I've also been told to not use a large envelope with "extra" stamps on it as it may be flagged.Seems that's what the terrorist's do when they send their crap in the mail.
The reason they don't want you doing this is because oversized envelopes need to be handled by hand. And if you take a business size envelope and place the a stamp in the upper right corner and then one below it the afcs will only cancel the stamp in the normal spot. In theory the other stamp can then be reused, causing a revenue loss. The afcs only looks for stamps in the upper right hand corner, put a stamp any where else and it will go into the reject bin. Now your letter will be ran thru the machine until it finds the stamp, which it will not, it gets shredded, which it might or depending on what software is in the machine it will just sort it and send it out. Actually the machine is not looking for a stamp it is looking for........better not say as it is the way around paying postage................