Where did ' cei ' come from ?
From the verb cael - it's the "you" form corresponding to the "I" form ca (here lenited as ga because it's a question).
This is contradictory for me.
May I pay. Is asking if I would be paying.
You may. Is saying the other person is paying.
It's probably more helpful to think of this as a conversation.
So A will be the one paying.
I though 'ga' have 'cei' were parts of the verb 'to have' for example 'ga I baned' = 'may I HAVE a cuppa'
Does it's use in this context translate roughly as
"can I have payment(For this)? You can have" ?