I only have to pay 19 bucks a month once...
no, you only have to pay $19 once, period. if you want to continue getting updates, then you have to keep paying. otherwise, you can cancel your subscription right after you pay the first time (although I would suggest waiting until the day before your next payment, just in case an update is released during that month). you can still sell your game even after that. the only restriction will be that you won't have access to later versions of the engine