Yes, content can be printed on the flap of the envelope, however, one of the double-sided printing options should be chosen. Commonly, the address or a slogan is printed on the flap, though a logo or simple design can also be used. The only constraints are the number of inks that can be used and the fact that the design can not have bleed. The standard margin guidelines should be taken into account for the flap as well as the rest of the design.

If there is content on the flap, it should be supplied as a second page in the PDF, not added to the top of the front artwork. This is because each side of the envelope is printed separately, as the envelopes are already manufactured.