It has recently been brought to my attention that I have been using ounces and fluid ounces interchangeably, even though they are definitely not the same thing at all.
A fluid ounce is a unit of volume equal to 1/128 of a gallon (US). It was originally the volume of one ounce of wine. An ounce, on the other hand, is a measure of mass that is equal to approximately 28 grams.
However, you'll probably be unshocked to learn that makeup products frequently do not have the exact same density as wine. Thus, a fluid ounce of any given makeup product will probably weigh substantially more than an ounce.
In conclusion: oops. I promise to be more diligent about properly labeling my terms in the future.