There are a couple ways to do this. We chose to just let them choose an existing package, whatever your cheapest business package is. then offer to turn it up when they have an outage to a faster plan for an additional fee. Could be the rate of the higher plan they choose and turn it back down when the outage passes.
Another option is to have them pay for usage, but that seems difficult to track if your billing system doesn't include it in the invoice automatically.
