Uncertain<T>

A person might have mistyped a price, a barcode may have been misread, the unit prices might be correct but the quantity could be mistaken. Modeling uncertainty well isn't just about measurement error from sensors.

I wonder what it'd look like to propagate this kind of uncertainty around. You might want to check the user's input against a representative distribution to see if it's unusual and, depending on the cost of an error vs the friction of asking, double-check the input.

Typos seem like a different type of error from physical tolerances, and one that would be really hard to reason about mathematically.