I always ask at the store before I make any purchase if I will get a Disney Visa discount at that store if my purchase is over $50 - that way I can confirm they participate before I get to the register.
As for the select merchandise, the only things I think I didn’t get a discount on were the reusable shopping bags they sold at each park and the actual personalization (custom wording, dates, artwork) on Christmas ornaments - other than that I have gotten the discount on all other merchandise including a Dooney & Bourke purse and wallet, t-shirts, PJs, artwork from Art of Disney, Jim Shore figurines, ornaments, baby clothes, kitchen stuff, animal figurines from AK, etc.