Thanks for the bug report! Does this happen with specifically "overweight" parcels or any parcel? If you could post a screenshot of the mistake preview when this happens, that'd be really helpful -- we've been trying to catch this bug for a while, but haven't been able to replicate it.
In terms of game balance, did you unlock the stamp pack upgrade and purchase mystery stamp packs? If it's not possible to price everything correctly with the upgrades purchased, we'll definitely have to tweak the pricing balance as well.