The best place to get postcards mailed from is inside the scrapbook store at Downtown Disney. They have one of those Pitney Bowes machines that will print the postage on the postcard so you don't need to bring stamps. They do not charge extra for the postage...just the actual cost.
The best part? This is the *only* place to get a Disney cancellation!! Every place else is stamped Lake Buena Vista since that's the local post office....even when you drop it in the mailbox on Main Street. When we were there in November and mailed out postcards for our upcoming trip, Tinkerbell was imprinted on every card with the postage and Magical Gatherings logo!! Very cool!! And they'll know you mailed them from Disney rather than from the grocery store!!
Just an extra touch of magic!!
Katie