I was in line at my local PO window one day. The lady in front of me was mailing something in one of those envelopes with the metal clasps. The PO worker waiting on her told her to never use the clasp envelopes to mail anything. She said they charge more postage for the clasp envelopes. She went on to say if you have an envelope with the clasp....just take some pliers or whatever & remove the clasp & it will save you a little on postage.
If a postal worker seems to be giving you a hard time about something don't argue with them. Just ask to speak with their supervisor instead. You'll be surprised at how quickly their attitude changes when you do!