A student joins mid-month — is the invoice the full amount?

Not necessarily. The first invoice of a monthly pack can be prorated — per remaining session or per day, depending on the school's setting — and the basis of that calculation is frozen as a note on the invoice, so it can still be explained months later.

The order of calculation is the same for every recurring invoice: prorating first, then automatic discounts, then the family wallet balance if it is used. Because the order is fixed, two admins cannot produce different figures for the same case.