Pay-On-the-Go Use Option

Add option for free/trial/premium users to continue using the platform once they are out of ressources within a POG (pay on the go) approach, and according to specific POG admin settings (price values, use-limits values, payment,...). consider to notify users with that fact (that they are about being converted) an 'x time' and an 'y minimumremainingressources' pre-set values before forcing them into POG users unless/till they renew/upgrade to prepaid or membership users. a admin can choose whether POG users can continue within a consumption approach (top-up balance) and/or within a creditation approach (periodic calculation and deduction approach), then select which payment gateways to be enabled for each of the two approaches.