Hello,
you can pay 19€ to start the subscription. That will grant you access to the UE4 toolset. If you don’t want to pay for future updates on a monthly basis, you can simply cancel your subscription. Of course, you won’t be able to update your engine and won’t have access to the sourcecode anymore, but you may still use it to create products and release them (even commercially).
The monthly payment fee is only for access to the sourcecode on GitHub and for any updates.
Whenever you want to update your engine version, you will have to reactivate the subscription though.
Check the FAQ for further questions: FAQ