Since the last rate change, which increased the complexity of calculating postal rates, I simply put my own postage on the 000 bubble wrap envelope and pay what I feel is the lowest possible rate. By lowest possible rate, I mean that I ignore the "Large Envelope" rate. So for up to a 2 ounce envelope the postage is 59c (42c plus 17c). For each additional ounce, I add 17c. I haven't had a package returned for postage due yet.