What happens when my allowance runs out?

The action is declined, with a message telling you which cap was hit. Nothing gets charged on top without you asking for it, and nothing gets purchased and billed after the fact.

The allowance is deducted before the external call, never after. That's what guarantees a network error can't turn into purchased data with no record, and that a cap is actually a cap.

To keep running within the same month, the path is upgrading your plan or buying the standalone module.