Couple of points here. It always seemed a bit rich that the PO deprived retailers of a large lump of their livelihood and then made them pay to preserve bits of the business that were deemed to be in competition. How does the PO know there are any bill payments taking place anyway? (It isn't even in the retailer's name).
Our retailer adds that removing the terminal would preserve nothing for the PO. "There are three other PayPoint terminals between here and the next post office and the Payzone I have doesn't do either Swalec or British Gas."