If your ads aren’t delivering or spending up to the daily budget, try to increase the billing threshold. (Medium)

The amount you're charged may be slightly more than the threshold if your account accrues costs very quickly. You can be charged multiple times a month if your account reaches your billing threshold repeatedly over that time. (Google Ads)

Again, like I said, this is often referred to as your spending limit, but the true term for it is your billing threshold. Facebook starts you off with a very low threshold, and over time as you make payments they keep increasing it, up into the tens of thousands. In fact, I have friends who have accounts with billing thresholds as high as $100,000, meaning that they can spend $100,000 on Facebook ads before they even see a bill. (New Tip Today)