Local post officer here in Southern NJ will automatically declare anything in a bubble mailer as a package. They will not budge from that stance. And in fact, if you were to mail me something in a bubble mailer that your post office accepted at another rate then MY post office would charge postage due on the difference from the rate you paid vs the package rate they think should have been paid. I've had this argument with the manager at the local post office many times and can't get anywhere.

But just a word to the wise, it can cost the recipient postage due even if you think you can mail it cheaper!

For slot cards, I switched to using a standard 6x9 inch manila envelope WITHOUT A CLASP and use some thin cardboard inside for protection. I can usually mail up to about 8 slot cards and still be under the 4 ounce limit for first class mail but with the non-machineable surcharge. The size is just under the limit for a large envelope and the key is to NOT have a clasp or it will automatically go to a package rate.

If you are mailing packages you might want to check out pirateship.com who offers the lowest commercial rates around with NO monthly fee. Sign up is free. Only drawback is that they ONLY handle USPS packages, no first class mail, etc.