If the drop-off doesn't work out or they need more stuff like that delivered, maybe try
https://fsastore.com/ although I don't know if they will have the same address issue or not. On Amazon, have you tried slightly altering now you type in the address? I can't remember what was going on with ours but I think it gave my parents some issue with our address before (even though they had typed it in the same way we have it in our Amazon account). I think they made a minor adjustment and got it to work but can't remember the exact details.
FSA store will let them use FSA or HSA funds but can also just use a regular credit card.