a) this is like stealing money from PP. You are not a friend and family and are using this service to circumvent a fee for a sale which this is. If you think it's ok to go to Chipotle and ask for a (free) water cup and then fill it with soft drink that's the same principle.
b) cash is best (usual and customary) you are trying to go against usual and customary for the benefit of the buyer (not you).