"Registration" is a concept that your vendor has coined; i.e. it is not a legal requirement regardless of the dollar amount and has no universally understood meaning.
At $500, a nonreloadable card is a poor money laundering vehicle unless it can be purchased in volume. If you have limits on the number you will sell per person per day, universal "registration" sounds like an excess of caution. In any case, no one is going to be able to show your management that registration is required by anything other than the vendor.
In this world you must be oh so smart or oh so pleasant. Well, for years I was smart. I recommend pleasant.