> If you don't, we're haggling over the price, not over the principle.
Sure, haggle away. I see no problem with this. I don't see the need for Heroku (or anyone for that matter) to support a user's desire to haggle. I do think, however, that a warning should be given if your usage surges suddenly so that you can react appropriately.
If you don't, we're haggling over the price, not over the principle.