Imperfective aspect is about state and perfective aspect is about state transition.
So if you are talking about the outcome of a single action ("please pay"), you would use perfective (пожалуйста, заплатите), because what you really want is to make an incomplete payment complete (state transition).
You could use imperfective here as well (пожалуйста, платите), but this would mean that you are more interested in seeing the person paying rather than obtaining the result. For instance, if a patron is waiting for a POS terminal to get fixed and you ask them to retry their payment, пожалуйста, платите would be the correct form.
If you were talking about the state of things ("please pay your bills on time"), you would use imperfective (пожалуйста, платите по счетам вовремя). This would mean "be a good person and organize your life so that you never have overdue debts".
If you said пожалуйста, заплатите по счетам вовремя, this would mean "there are a bunch of bills on your desk, please, at least this one single time, make them paid before they expire"