[mothered]

Whenever I order something from amazon, that is clearly below 120 grams, it always shows the weight on the package, such as 0.1kg, 0.2kg, 0.3kg, etc. Literally every package I've ordered has the weight on it

That's defined as the "actual weight" - product weight and packaging In round figures.

Carriers use the actual weight, the dimensional weight (package length x width x height), and the billable weight (the greater of the actual & dimensional weight) to record the total weight and for cost of freight. Using that formula, the majority of shipping scales do not Increment the 120 gram bracket, which Is why It (120 grams) has a proven track record of success.