I'd just tell the police that I trade tobacco a lot :P I have a digital scale specifically for trades that I purchased before I made my first one on here (I try to think of everything in advance). I just like to be fair, and I am terrible at guessing, it just never sat right with me. Some people are better at it than others; personally, I like having the piece of mind that whatever I have stated is in the bag, is in the bag. It also helps a lot when mixing differint blends together to always get the porportions right, which I sometimes do in my experiments when a blend just isn't quite right.
Buying a postal scale would be a good idea if your into shipping items via USPS a lot and don't want to waste your time in their office... You can get them on Ebay for $18, free ship. Appaerntly they are accurate within .1oz for the first 0-25 pounds.