Subscription Customers Booking & Checkout

[Tash2]

I am a spray tan artist. I’d like to create a monthly membership, using the subscription. They get charged a certain amount for multiple tans during that month. 
My question is, say a customer purchases a monthly 3 tan subscription. Now that customer goes and books a members only tan. Since they already paid, after their service how do I check them out? 

does this make sense? 
do I create a special service named members only? 

[JustinC]

So to be clear, are you trying to track/checkout each individual tanning session for a tanning package already purchased by your client? Sorry if I'm off target here but I want to make sure I'm understanding. 

@Tash2