[timjones]

Interesting. So if their 12-month plan allows for 100 seats, and they grow to 150 seats during those 12 months, then for month 13 they would:

1. Pay a pro-rated amount for exceeding the seat limit during the first 12 months.

2. Pay the 12-month contract rate for 150 seats (or maybe more if they expect to continue to grow)

Is this a common practice?

[thaumasiotes]

I have no relevant experience, but I felt I should note that I would be extremely offended by being billed retroactively.

Hopefully you would provide some notification at the time they exceed their limit.

[TheAdamAndChe]

I second this. Instead of billing retroactively, consider just not billing for the extra seats during that period. Maybe notify the user that they've entered a grace period for this billing cycle, but starting next cycle they can expect to get charged for the extra seats.