[thaumasiotes]

Nonsense. If you're going to bill for the alert, send it when the charge is the difference between the current bill and the limit.

Of course, it's no more realistic to charge people for the alert than it is to charge them for email receipts. Do you not get those from Amazon?

You implement the feature cap by ceasing to provide the feature once the account is over the billing limit. It is always possible to project the cost of "freeze everything until the end of the month", because there is no unknown information in that scenario. Nothing you've listed is an actual problem.