Hey Archie.....just to push the limits of your clerk......tape all edges of your flat rate envelope but do not tape over the self adhesive part of the envelope which is used to close it and then see what he says.....as you will not be using the tape as a means of closure. This will still provide most of the much needed support and security for the envelope with out violating the statement on your envelope.

If he persists in saying you can't use tape, ask to see the regulation where it specifically says that you cannot. If he says it "modifies" the package, have him show you the definition of "modify" in the postal regulations.