I've always used my UHC card to make OTC purchases, and I also noticed that (this year) the products came (very quickly) from Amazon (which I have no problem with, btw).
I just set a reminder on my Google Calendar on the first day of each quarter, and I place my order as close to the last cent as possible (LOL).
It's a cool and convenient benefit.
Oh, and the balance does NOT carryover - Use it, or lose it, each quarter.