Charge a monthly subscription fee AND a per class fee?

Hi all. 

I run a fitness website where clients in my area sign up for outdoor fitness classes. We charge a recurring monthly fee and a per class fee. So if a client books zero classes in a month they are charged $20. 1 class: $20 + $8. 4 classes: $20 + ($8 x 4). Etc. 

I am unsure of how to do this (or even if I can do this) in the new squarespace site that were building.

Does anyone know of a way to charge a recurring monthly fee (which occurs monthly obviously) as well as a per class fee via the appointment-setter for each class an individual schedules. 

Please do not just tell me to average it out because the monthly fee is a psychological tool that leads to significantly more engagement and revenue. We learned this the hard way. 

Would I create a  membership area (with a monthly fee) and then put the scheduling page (with per class fee) behind  the membership paywall? 

Any and all help is greatly appreciated! 

11 hours ago, AllOut said:

Would I create a  membership area (with a monthly fee) and then put the scheduling page (with per class fee) behind  the membership paywall? 

Yes.