There's no industry standard around this, this is basically a negotiation, with the appropriate considerations for how much leverage you or they have. We've had situations where our SaaS vendor was too big and we were too dependent on them to pick a fight over back-charges, so we paid. And there were other vendors with whom we raised enough of a stink around this that they relented, because they were not an essential dependency and we could switch.

How essential are you to the operations of your customer?