Thanks for the feedback, if you are consistently underpaying, chances are that going down a row on the price chart will solve your problem. The chart is a bit confusing and we'll be updating it soon.
The "up to weight (g)" column should be more clear with the ranges: the "10 g" cell should read 0 -10 g, "20 g" should read 10.1 - 20 g, etc. So for example, a package weighing 25 g going to Tator Isles would fall in the row marked "100 g" since it's over 20 g and fits in the "up to 100 g" range, and it'd need $0.50 postage to ship.
You can use the scale to do the stamp math :D -- hope that helps!