That's our bad, the pricing chart really isn't designed well and we'll be updating it in the next build (hopefully) :) 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.
Hope that clears things up and thanks for the feedback!