Because you'd prefer to pay later? (2,7)
I believe the answer is:
on account
I'm afraid I can't even guess how this answer relates to the clue.
Can you help me to learn more?
(Other definitions for on account that I've seen before include "To be paid" , "an instalment" , "As partial payment with the rest to come later" , "Involving part payment" , "By deferred payment" .)