Allow users to configure storage to deduct the cost of keeping it until the end of the billing cycle from their spending limit. Then even when you hit your limit, you still actually have enough allowed spend left to pay for your storage. Actually the same principle could be used with compute or any other per-time service, and could just be an option when creating a resource. If you ask for a reservation, then failure to make it because it would exceed your spend limit results in failure to create the resource.

This is a basic problem that every adult needs to know how to solve, like "how can I make sure I don't run out of money to pay for food and shelter when I'm buying toys?" You set aside money to pay for the important things first, and what remains sets your limit for discretionary spending.