I build dulcimers in a humid coastal climate and I do worry about shipping a dulcimer to very dry climates. So far I've had no complaints. I  always use aged dry wood to start with and that helps.

All instruments need a time to acclimate to there new home.

I would be more concerned if the dulcimer has wood pegs. Wood pegs are like barometers when the climate dries I  know  because I'll hear strings popping in the night!. 

Back in the sixties during the guitar boom, made in Japan imported instruments came with a thick plastic like finish to survive the ocean crossing. Which is why they sounded so bad.

Today imports are shipped climate controlled.

I would say start with good aged wood and a good thin oil finish and cut your fret slots with a saw intended for fret tang dimensions  and you'll be ok... Robert 

I agree with Lisa and Ken that the instrument needs to adapt to it's environment. In fact I think that more humidtiy is better than the opposite: One of my gutiars lost it's bridge due to the winter room climate (coming from the gas oven). The luthier, who fixed it explained to me, that wood continues to "work" (means changing depending on it's environment). He even gave me a handout exlaining how different degrees of humidity influencing the wood:  Although it's for guitars I guess it's not much different for dulcimers, it's still a plucked wood instrument too :)

I will try to give the most important things from it (Text is from Tobias Ahlke Luthier at Essence Guitars, Oberwinter-Remagen, Germany, translated with DeepL by me). If somebody here understands German I can also post a scan from it.
Although it might sound a bit scary I also agree with Ken that you are overthinking it. Even if the dulcimer might suffer a bit from the climate at your sisters place, it should be fixable and in the worst case  she can always get a new instrument. But now to the wise words of Mr. Ahlke:

"Wood is hygroscopic. This means that even after decades of storage and in the installed state, it can still absorb moisture from the air or release it into the air. If it absorbs moisture, it increases in dimensions; if it releases moisture, it shrinks.  

The ideal humidity for guitars made of solid wood is about 50% relative humidity at a normal room temperature of about 21%. The woods of your guitar are stored and processed in my workshop at a controlled humidity between 45% and 50%. In this range your instrument can be played great and sounds best. If the humidity deviates, typical symptoms quickly appear and massive irrversible damage can occur:

• 60% relative humidity and above: The  string action  may increase, the curvatures of the top and back are exaggerated, glue bindings may break, frets and tuners are dull and tarnished, uncoated strings oxidize excessively fast.
• 40% relative humidity and below: The fingerboard shrinks, which is noticeable protruding fret edges. If necessary, the string action decreases. The wood is under tension
• 35% relative humidity and below: The grain of the wood is clearly formed through the varnish (e.g. the ceiling looks streaky). Cracks may appear in the ceiling and floor
• 30% relative humidity and below: Glue joints can break, cracks become larger and larger, the statics of the instrument are in danger!
• 25% relative humidity: glue joints can break, frets become loose, the instrument unplayable, maybe forever!

Humidity can be easily controlled with commercially available electric hygrometers, which are either stored in the instrument's case or placed at the instrument's location. If you do not want to or cannot regulate the humidity in the whole room, it is recommended to place a humidifier or dehumidifier near the instrument in the closed head. From our point of view, the "Humidipak" from D'Addario/Planet Waves has proven itself here, as this can keep the relative humidity constant at 48%, regardless of whether humidification or dehumidification is required.

Climate and temperature

A guitar feels at home where the player also feels at home: Neither does one like to sit in the blazing sunlight, in the middle of a draft, nor in front of the turned-up heater or in a damp cellar.
Temperatures between 0 and 40 degrees Celsius (32 - 104 ° F) are usually no problem for your guitar, although you should make sure that the guitar can slowly get used to the new temperature. Otherwise, fine cracks may appear in the wood and varnish and glue joints may be damaged.
40 degrees Celsisus (104 °F) and above: the woods begin to bend with the string tension, deformation occurs. The varnish may soften, feel sticky, or develop pressure marks under light loads.
0 degrees Celsius (32 ° F) and below: wood, varnish and glue joints are cold, hard and brittle, cracks and breaks are likely to occur with stress."

So I think a high humidity might less a problem than the other extrems (lower humidity and to much heat): At least the potential results sounds fixable. If the climate is really to much for the dulcimers storing them in a case with a Humidipak or a simmiliar system ( https://www.daddario.com/products/accessories/humidification/automatic-humidipak/humidipak-maintain/ ) should help. Note: As said above this tool and the hints from Mr Ahlke are for guitars, but since dulcimers are made of wood too, I thought, they might be helpful :) 

If somebody asks for it I can scan Mr Ahlkes handout.

Regards, Jost.

Ken please stop 'yelling' or I might have to send someone to remove your CAPS lock and exclamation point keys in the middle of the night. Seriously, bro.

Nate, I'm not sure why you'd be trying to reduce humidity during shipping... the dulcimer is going to be all wrapped up in multiple layers during transit and also... what good will it do once the package is opened anyway?  It's true that the instrument is going to have to adapt to its environment one way or another. Just my opinion, but right by the sea I don't see how silica or gel bags will provide longterm relief unless the dulcimer spends almost all its time in its case.

They say that it's rapid drastic changes in temps and humidity that harm instruments more than simply a constant environment that's hot, or humid, etc.

I have a good friend who learned the hard way. They forgot their beautiful Martin guitar in the car for an hour on a very hot summer day. Inside the car was over 100F. They rushed it into an air conditioned house and immediately opened the case to check on it. It looked ok. Within one minute that guitar in its open case literally exploded with a huge bang, into a million pieces.   If only my friend had brought the case into the cool house and just left it there for several hours to adjust slowly prior to opening the case , the guitar would probably have been ok. But I digress...

I honestly don't know much about wood, but maybe using hardwoods rather than softwoods in construction will help keep a humid dulcimer more stable? Or perhaps making the back piece a tiny bit thicker to increase structural stability? Just random thoughts.

Banjo players, who struggle mightily with humidity if they have real calf or goatskin heads... sometimes use the old trick of spraying two very light coats of Aquanet hairspray (or one of those art pastel fixative sprays) on both sides of the skin head. This actually does help prevent the skin from absorbing so much humidity from the air that it softens and sags and makes the banjo unplayable. This has worked remarkably well for me on my real skin banjo heads during some very humid summer camping weekends. It doesn't seem to effect the sound either. Makes the skin a little more stiff and waterproof. It does darken the skin a little and makes it more translucent. (which i find attractive)
I see no harm at all in spraying 2 light coats of Aquanet on the inner sides of your dulcimer body wood. We're not talking about a Stradivarius here. It may actually help seal and prevent the wood from expanding/contracting quite as much due to fluctuations in air moisture. I suppose you could spray the outer surface too, but it may create an unwanted finish so best to test first.

 YOU ARE SERIOUSLY OVER-THINKING THIS WHOLE THING!

"-coating the inside of the dulcimer in something water protectant: an idea I had earlier that seemed to not be useful in general, but maybe in this context would help it hold up."

No, NO, NO!   I live much closer to the ocean than your sister, and coating the inside will NOT work.  If it did I would be doing it.  For two years I lived less than 100 yards from the ocean on an island in the Pacific near the equator, one of the saltiest environments on the planet!  If coating the inside would have worked I would have done it.  IT DOES NOTHING!

-gluing in frets with a strong glue: in general I know that stronger glues tend to make maintenance and repair much more difficult, but if I want to build it to last, maybe I should glue them in with something heavy duty to reduce the chances of them shifting?

It is worth noting that I have traveled down there with dulcimers I have built and always had to pummel some raised frets every time I got there, so this is a primary concern for me.

You should not need to glue frets in if your slots are the proper width (not too wide), cut to the proper depth (not too shallow), and you have hammered/pressed them in properly.  However, if you feel you must glue, use one of the slow setting Super Glues, as my friend John Knopf recommends will do the job.

Gel Packs -- WILL NOT WORK. Not the way you think. You'd need ten pounds of silica gel and it will only last a few days. 

The fact is that the Dulcimer needs to adjust to the environment where it will live.  If it can't because you sealed it up inside and out, one scratch will cause it to warp horribly.   If the dulcimer can't adapt to local conditions because you artificially adjust it's local humidity with gel packs, it will warp the instant you let the humidity change.

Nate, you could tape some silica-gel packets to a string, so she can pull them out of a soundhole when she gets it.

I've used super-glue to glue in frets, and it works fine.  Should you ever need to remove a glued-in fret, just hold the tip of a small soldering pencil on the center of the fret, and the glue softens in a few seconds so you can pry it out..

Thank you all very much for your advice. First of all let me just say that my sister has TONS of pets and so she keeps her windows open all day every day and lives only about 2 miles from the pacific ocean, so the salty air is ever present there, whereas I am currently inland in southern oregon where the weather is similar, but FAR less humid. It sounds to me like I am overthinking it. With that in mind I'd like your perspectives on a few ideas i think might help.
-silica packs in the dulcimer: I'll be the first to tell you my understanding of the proper application of silica packs is limited to the shoes and jerky I've bought, but I believe it may help to keep the dulcimer intact during travel. If thats the case id like to place the soundhole in such a way that she can retrieve and packs out of it
-coating the inside of the dulcimer in something water protectant: an idea I had earlier that seemed to not be useful in general, but maybe in this context would help it hold up.
-gluing in frets with a strong glue: in general I know that stronger glues tend to make maintenance and repair much more difficult, but if I want to build it to last, maybe I should glue them in with something heavy duty to reduce the chances of them shifting?

It is worth noting that I have traveled down there with dulcimers I have built and always had to pummel some raised frets every time I got there, so this is a primary concern for me

I'd love more input! Stay well y'all
-Nate

I really don't think this a big issue. People along the Pacific coast have played stringed instruments for many years. People buy and play lots of acoustic guitars from inexpensive Asian models to high end Taylors and Martins. These instruments receive no extra treatment for humidity control. The outsides are finished and the insides are not. Build her a dulcimer. The general rule of thumb is that if you are comfortable, the dulcimer will be comfortable.

Ken

"The dulcimer sings a sweet song."

I live in SW Florida "right next to" the Gulf of Mexico.  I fact I lived on a boat here for 10 years.   I also used to live on an island on the equator in the far Pacific where the air is reallllllly salty.  Never had any issues from that.  Of all the dulcimers I've built I only ever had issues with frets raising because I lived where it was dry and the recipient lived where it was damp.  

To put it simply, you cannot, just cannot build a wooden instrument and seal it completely against humidity.   A normally made dulcimer will not rot/degrade due to ambient environmental conditions.  However, hurricanes. typhoons and floods are not "ambient environmental conditions".  

How wonderful of you to build your sister a dulcimer. I would think if she has a reasonably "tight" house and does not live with the windows open all the time, the dulcimer may be protected enough kept inside. As exciting as it sounds to sit by the beach to  play I probably would not do that. I live in NC with 95% humidity much of the summer with no problem imparted to my dulcimers kept inside the house.

Hello all. My sister loves string instruments but has never been able to learn any and when I let her play my dulcimer she had a ton of fun, even pulling it out several times to show friends the couple of beginner songs I showed her. I'd really like to build her one, however she lives in northern California right next to the ocean, so I am very worried about two things:
1 Sending it to her and frets raising making it unplayable when it gets to her
2 It degrading and warping extra fast due to the salty ocean air where she lives.
I would love any and all advice that might help with either or both of these issues.
Thanks y'all for all the help over time
-Nate