Best thing to do is simply bring the package to the post office. They will personally weigh it to make sure it is accurate, and tell you exactly how much postage would cost. You give them the $$, and they will take care of putting the correct postage on the package for you.
You can do what other people have suggested (slap 5 41-cent stamps on yourself) but I don't like doing that because you will spend more than you need to (and the post office rips us off ENOUGH for postage already)!
Besides, this way the weight is guaranteed to be accurate! If your measurement of the weight is slightly different than the PO's, you will pay for it. If they measure slightly MORE than you, they will send the whole package back to you and demand the extra money - even if it's one penny! If they measure slightly LESS than you, you paid more for postage than you should have - and they won't even THINK of sending you a refund!!