In most cases payments are automated and the client isn’t really meant to ”top up” midterm or stagger payments. Kind of like you can’t pay for more than one term of say Netflix in one go or while current term is active. In some cases correct approach may be to not allow staggered payments because webmasters may plan a pricing change, or allow term update when current term is pro-rated and cancelled if a new one is started, or perhaps a limited number of “units” of time can be purchased for each subscription. I am not particularly sure which is better and not very keen on creating a massive set of settings for all options, so I think we still need to discuss.