Or don't. Just ask users to tell you (without any burden of proof) if they've been paying less. Will a handful of users abuse this? Probably. My bet is that the

Or don't. Just ask users to tell you (without any burden of proof) if they've been paying less. Will a handful of users abuse this? Probably. My bet is that the vast majority are honest people -- particularly if the circumstances are clearly and simply communicated -- and will not take undue advantage. The upside is that you won't be pissing off the honest majority, and the cost is probably (admittedly unprovably) small.