"M" is for M-Bag (International Shipping)
Do not use post office boxes labeled "Priority Mail" or "Express Mail". You have to either use brown boxes, or boxes covered with heavy brown paper. Your parcels should be free of extraneous markings, so if you are recycling somebody else's boxes, re-wrap everything in brown paper and be sure to put your return address with a notice "RETURN POSTAGE GUARANTEED" in the rare event that the paper becomes separated from your parcel.
Pack the parcel securely as you would for any international shipment going surface mail. We invariably box the books, rather than bag them. Fill out the two forms that you need: PS 2976 which is secured to the right of the address on the parcel, and PS Tag 158, which is handed in separately. Your address and the customer's address go on the customs slip, the M-bag tag, AND on the parcel itself. Please note that if you are sending more than one M-bag to the same customer, you can link the sacks by putting a correct fractional mark 1/5 (1 of 5), 2/5, 3/5, 4/5, etc. for a 5-bag load on every tag. You can request a Certificate of Mailing, if you wish, when you are checking in your parcel. Used properly and with precautions (like third-party insurance), M-bag service is a great way to expand your business to the rest of the world.