Hi, I've not shipped to WDW but I do a lot of shipping for E-bay (700 items and counting).
Things to remember when shipping items:
The UPS store does not provide free boxes - you have to pay for them and they are expensive. I bought a box and paid $7.
The shipping for that box cost me $18 from MA to KY there were 26 pieces of kids size 7 clothing in it.
Really, it's probably not all that much more convenient; schlep to the store to buy the box, wait in line to get the box, schlep it home, pack it, schlep it back to the UPS store, wait in line again to ship it, schlep home again.
You have to time this just right to get your stuff to WDW in time for your arrival but still have it at home when you need it.
Keep in mind UPS does not deliver on Sundays or holidays and charges extra for Saturday delivery.
You still have to go to the airport to schlep yourself to WDW.
Don't forget to add the cost of tape to the box (@$3) and don't forget to bring it with you to WDW. (Will they allow this on the plane as a carry on? And if not can you buy it at the resort?)
Boxes over a certain dimension carry a shipping surcharge and can take longer to arrive.
Because you have to ship it several days before you leave, and some items are forbidden in your carry-on, you might need to have extra personal care items for use at home after you ship your items.
Just like airports there is a list of forbidden items for shippers as well - make sure you check this before you ship - some shippers don't allow shipment of batteries or items with batteries in them, some don't allow liquids, others don't allow food, etc.
UPS rates include insurance up to $100 on the box. For UPS extra insurance costs extra money. Make sure you add up how much your clothes worth and how much the insurance will cost when pricing the shipping.
HTH
ETA - As I found to my regret when my fruit got frozen and turned to mush - UPS items can sit in hot warehouses or hot trucks or cold trucks or cold loading docks for overnight (or longer) make sure your items can withstand the temperature extremes because the backs of the trucks and the warehouses loading docks do not have heating/airconditioning.