1) WDW cannot "postmark" the mail, other than Lake Buena Vista.
. . . Disney is not an official postal designation
. . . the Post Office on property is a normal post office
. . . thus, the postmark is for the ZIP code, and no special Disney marking
2) But, GS at MK can put a WDW-type rubber stamp next to the postmark.
3) As for mailing/sending things from resort
. . . each resort has a mail slot near the Front Desk
. . . take the box(es) to Concierge who will seal them
. . . Concierge can send USPS-FedEx-UPS *
* You get an empty cardboard box from Gift Shop. Pack your goods.
Give box to Concierge and tell them you sipping method. Your room
will be charged the shipping cost. NOTE: There is no surcharge for
this shipping, just the actual shipping cost - unlike the stores where
they have a markup on shipping as a profit item.