This is a reply I just received from the Shop Disney Chat Room:
Please download the My Disney Experience App (Listed as Disney World App) >> Click on the 3 bars at the very bottom right of the main page >> Select “Shop Merchandise” tile to be taken to the Shop Disney Parks site. (Discounts are automatically applied when you are logged in through your connected AP/DVC Account on My Disney Experience. There are NO promotion codes to apply).
Also, they mentioned that you cannot do this on the MDE site on line......... "it is messed up"