I use Google sheet to record my revenue for months, but recently I have some trouble.

I offer both post-paid and prepaid service, so I have some customers prepaid for more than one month, while post-paid service would be charged each month because it's usage-based. In short, I have to distinguish prepaid and postpaid service, and the billing date, then sum the total amount I need to bill my customers.

I used to use the formula that simply multiply the prepaid service amount of the first month by payment cycle (how many months he prepaid), but recently it starts to go wrong because the amount would be different in each month.

I do search for some articles about sum the dynamic range, but soonly find it hard to use because I have to select my column conditionally, and even with mutiple columns to sum.

The sample of my sheet: