How buying works
Two different things are sold here, and they are paid for differently.
Packs in the Shop are Risor's own. You pay at checkout and the files are delivered to you — the order and its download live in your account.
Listings are offers from creators. Nothing on a listing is bought on this site. You talk to the creator, agree the work, and register it as a contract; you pay the creator directly, however the two of you agree.
A registered contract gives you a record neither side can edit and an arbiter if it goes wrong. A deal made outside one has neither.
Prices on a listing are the creator's asking price. A Robux figure, when there is one, is the creator's own number for the same offer — there is no way to pay in Robux here.