Can I put a bubble mailer flat rate label on a traditional paper flat rate envelope? I have had USPS get mad at me for using paper flat rate envelopes to ship 3d/ bulging shapes, so I switched to the poly mailers.  
  
But now I'm out of poly mailers and I need to ship some pants. Can I buy the more expensive poly mailer rate, and put that label on a bulging paper flat rate envelope? At least until the poly bags arrive in the mail.