Depending upon the counter clerk available to me when I do my flat rate mailings ... sometimes I'm told that I can't use any tape other than the self stick strip that is on the mailing envelope. I'm told thats the rules. Sometimes I get away with it, sometimes I'm not able to and I have to discard the old envelope and use a new envelope without affixing extra tape. See the notation I have circled on the envelope in my scan.