What happens when I use up all my upload units?

It depends on what plan you are currently on. Any of the premium plans will continue service without interruptions and you will be billed proportionally to how much resource is used over the set limit. For example, for Growth plan, each additional 1000 uploads costs  $1.25. For the full price list please visit the following page https://uploadcare.com/pricing/#features.  Free plan has hard limits, that is - your uploads are blocked until the end of the usage month when your credits are replenished, or until the upgrade to a premium plan.