There was a second after the question that girl!Roxas actually considered taking Axel up and saying that it would be ~wonderful~ if he bought the clothes. But then the remainder of boy!Roxas' mind made a comeback and kicked away those thoughts.
He...couldn't let Axel do that. He also wondered if he should maybe tell Axel that the kid's family he replaced was pretty rich.
"It's okay." It would only make things a little more awkward if he let Axel buy him clothes. Especially girl clothes.
When the total was rung up and said, the boy (girl?) reached into the pocket of his pants and paid via a credit card. If Axel wanted to question it, he could, and Roxas would explain.
But as soon as it was all done and paid for, the extra slip of paper for the store signed with his signature, he reached out for Axel's wrist (it was more of a without-much-thought action) and proceeded to pull him along and out of the store with his bagged merchandise in the other hand.