Hi, the main thing that you need is :
1. Define the default tariff with price 0 and speed 512/512
2. Setup client with this tariff. Customer should be in prepaid mode
3. You need the option for self-changing of tariff by customer. This will be available in 1.3 version, launched during next month
4. Customer choses the plan on his billing section. Also pays amount of money that he wants to pay via paypal or creditcard. For example for 3 weeks for 10Mbps. And his plan changes after receiving payment to that. Probably assistance is needed from our side to make 4th step work.
5. When customer balance is 0 after 3 weeks of usage, he is blocked. And you should configure blocking rule which in fact will limit his speed to 512/512. Then he can reactivate the plan he was using last time or change it.
So, everything is possible, but I'm sure that this setup is not possible to achieve with free splynx license without our assistance.