Yes SambaPOS does not support three digit decimals for inventory numbers. It is easy to support however I don’t know if it is a good idea to track cost for something costs 0.3 cent. Even coffee prices doubles in a year your cup cost won’t increase more than a few cents.
Like all POS software does this inventory tracking method migrated from factories but kitchen works different than factories. For this reason we implemented updating physical inventory by hand on end of day records and distribute cost difference to all portions that contains updated inventory item. That solves a lot of problems but of course we can improve it more for real life needs. I don’t think what we need to do is supporting 3 decimal digits. You need to know how much a cup of coffee costs and how many cups you can produce with items on hand. We need to find a way to track it without dealing with half grams or cents.
To solve your problem you can think changing coffee transaction unit to kilograms.