You added a decimal - ceil($73.28 * 10%) = $8. I’m sure you knew that and just typed it wrong, but it does speak to how easy it is to make a mistake with simple math, especially for people unlikely to frequent HN.

Case in point, I asked for 1/3 lb of deli meat yesterday at the grocery store, and received 0.41 lbs. Worse, the worker was concerned that they hadn’t given me enough.