That is so true ! Would you believe I've had china and pottery items sent to me in the past from sellers over there that were just packed in a jiffy bag and arrived in a thousand pieces !!!
I was lucky Penni, when my Post Office shut down I managed to buy their old parcel scales cheaply. They were the old type, not the digital variety, but it still gives me a fairly good idea of what the estimated weight will be with a cardboard box. If it's close to a weight band then I quote the higher price and will refund the difference if I can get it to go under... better to be safe than sorry because it's quite a difference in price between the weight restrictions when posting overseas.
I don't sell many small items but for the ones I do I use digital kitchen scales and tend to guess, using the 'DVD in a jiffy bag' method mentioned before, if it's going to be a packet or large letter.