That can happen if you have your database set to Int and you are trying to store something with decimal places. You need to make sure your database column type is float or double to allow decimals.
This isn't the case for the cart though, since no database should be used.
If you are still having problems, please post back and I'll open an incident for you so that an engineer can call and work through your problems with you over the phone.
If you update the cart show decimals value, and upload the files I think it should resolve the problem.