https://pe.usps.com/text/DMM300/604.htm#1080435)Use the above link for the explanation in the Domestic Mail Manual and print a copy to show to your postmaster.
You may have to go to a different post office where they are less difficult?
Also, if you use the correct nomenclature ( Mailer's Postmark Permit ) rather than MPP, you can search this forum for lots of posts about this subject
Peter