In previous posts, several ideas have been presented. Here is one more.
Idea 6: letter balance
When you need to measure the weight of something, e.g. the weight of a letter to determine the price of the poststamp, you can use an old fashioned letter balance. When your modular phone is equipped with the right module, your phone can determine the weight the letter and the price of the stamp based on the weight / postal tariffs.
The module contains a mechanical part that can elevate a part of the back of the modular phone. The modular phone can be positioned in such a way, that all weight of the phone is carried by this module and measured by a weight sensor in the module. After calibration (when nothing is placed on top of the phone, the module/app should indicate zero gram), everything placed on top of the phone can be measured. And if needed, the app can determine a price for the measured quantity (either a letter, some powder, or whatever).