This is the full pricechart and it happens whenever I launch the game(I use the browser), using the gilded postage I can still send them with the right price. But else it'll say that it got underpaid.
I also had another bug where with some packages I needed to pay 1.75 for postage, but I couldn't find it anywhere where it said that I had to pay that. Couldn't recreate that bug though!
And maybe it is fun to include for example postage collectors, who will buy postages from you for the amount that they are worth(so the very big ones for 4 and the small cube ones for 25 cents!