Buying add-ons during the current billing cycle

In this case, for example, your customer's current billing is $49/month from 1st May to 30th May, and then your customer buys an extra 1 member on 15th May at $10/month.

So we will calculate the actual cost of the add ons in the first month, and then sync the fixed cost from the 2nd month.

So the first month, the cost for extra 1 member is $10*15/30 = $5 per month. So the cost for the first month will be $49+$5=$54/month.

And the 2nd month's billing will be $49+$10=$59 per month.

Hope this is clear to you.