Hey Christina!
I really like to do this!
There are a couple of ways to go about this...
One way is I ship a box of stuff to the resort that I don't want to lug on the plane in the first place, like snacks and toiletries...things I won't be bringing home. But then I have that box at the Resort so I can use it to ship souvenirs back home. I've made sure to include packing tape and return shipping labels. At the end of my trip I put in souvenirs and give it it to either the Front Desk, Concierge, or Business Center's mail spot.
The last time I did this I used a good sized box, roughly 18" by 24" and it only cost about $12.95 for me to ship home. Of course that cost will fluctuate with weight and whatnot, but I wanted to give you an idea of what it cost me a few months ago with my box of souvenirs!
Another way to get souvenirs home is at each gift shop in Disney World you have the option when checking out to ask the Cast Member to ship them to your home address. Isn't that great!? The last time I inquired, the fee began around $8.95. That fee could definitely be different because it's been a while since I asked!
I have also ended up purchasing a new duffel bag at the last minute to bring home souvenirs! Can we say impulse shopper?
Have a great trip!!