Edit: What is an issue number and why does Get Games hate my money
Issue numbers are used for certain debit cards ie Switch and Solo cards in Europe. If your card doesn't have one just leave it blank.
Get Games does have a weird merchant system. I've been sent Steam codes before for a purchase that did not complete but were unable to be redeemed due to the transaction failing. I emailed their support and apparently that was the way they handled out of stock keys. Hope it's changed now but they used to be a funny old bunch.