Additionally please research decimal floating points before disagreeing.
For what it's worth, values on the order of 10^18 are exactly representable in int64; decimal64, however, only has 16 significand digits, so there you are already affected by rounding, and it's not far from where the distance between consecutive numbers is greater than 1.
In general, with floating points, you either waste lots of space for excessive precision around zero, or you quickly lose precision when numbers grow. On top of that, you get all the other quirks of floating point numbers such as cancellation.
Or you do have a gram price and are selling literally thousands of tons to a big business
I'm imagining it, and I don't see why storage or computation time would be major obstacles to a calculation you run once a year. Use whatever big integers you want?
Suppose we dedicated one $100 hard drive (per year!) to storing all the relevant data. I feel like there would be plenty of space left over, and the budget would cover it.