If you seriously considering monetizing this service there is pretty much one licensing option: AGPLv3
Affero GPL will guarantee that nobody will simply take all your hardwork, setup couple of payment gateways and start getting $$ without giving you anything.
If you go with AGPLv3 then anyone who is using your code will be forced to behave in fair way: contribute all the changes back even if they just install it and offer as a SaaS.
If you seriously considering monetizing this service there is pretty much one licensing option: AGPLv3
Affero GPL will guarantee that nobody will simply take all your hardwork, setup couple of payment gateways and start getting $$ without giving you anything.
If you go with AGPLv3 then anyone who is using your code will be forced to behave in fair way: contribute all the changes back even if they just install it and offer as a SaaS.