The Post Office (www.usps.gov) has a postage calculator that will calculate postage with all options by weight - no charge.
Stamps are much like counting loose change. As for myself, I don't have that many odd denominations of stamps laying around and I learned how to count change in elementary school. I hope they are still teaching that skill.
The question one should ask themselves is: how much money will I waste in excess postage, if I miscalculate, whenever I actually use odd denomination's of real stamps, before I reach the $9 fee for this program?
In other words, will my losses, should I make a mistake ever exceed the cost of this program?