Sirius explains this in his letter to Harry right at the end of PoA. Crookshanks took the order to the Owl Office for himi, then he used Harry's name, but told them to take the gold from his vault in Gringotts - vault 711. If anything I've marvelled at that vault being only two vaults from 713 - the vault where the Philosopher's Stone was stowed before Hagrid fetched it in book 1.