Confused on billing threshold for FB ads

Received an email for a bill for FB ads. Is said I was being billed two dollars for reaching by billing threshold, with 72 impressions. The campaign is set for 1 week with a daily limit of Twenty dollars.
Why is it charging me just 2 dollars? Why didn’t it run up to $20? I did not see any setting for a 2 dollars threshold when I set up that ad. What am I missing here?14%20PM