how to print return address on flap of envelope

Whether you can do this or not will depend on how cooperative your printer is. I would guess that you are more likely to be successful if your printer is one that feeds envelopes crosswise rather than lengthwise, but in either case, what you would have to do is set up the return address in the top center of the envelope and then feed the envelope upside-down (with the flap on the side that will be printed).

If your printer takes envelopes crosswise, you are more likely to have success if you set up a document such that you can feed the envelope bottom edge first, with the flap open (the flap is less likely to be damaged--or the print crooked--if it's the trailing edge).

Even commercial printers hate to print return addresses on the flap because the paper handling is so difficult, but they do print them with the flap open, feeding the bulk of the envelope first.

I was assuming two passes in any case, if the OP wants to print a delivery address as well. What I was envisioning was printing a supply of envelopes with return address on the flap (as opposed to having them commercially printed).

By and large, the desire to have the address on the flap often arises with formal invitations, such as wedding invitations. For this purpose, a suitable (and perhaps superior) alternative is to emboss the address. A hand embosser can be purchased that permits interchangeable plates, and the plates can be rotated to emboss either envelope flaps or the letterhead of stationery. Hand-embossing the envelopes is time-consuming, of course (if a great quantity of invitations is purchased, it's probably more practical to have the envelopes commercially embossed), but less nerve-racking than mangling expensive envelopes in an intractable printer.

If you run the envelope through crosswise, with the flap open, you should be able to print both address and return address in one pass. Some creative page sizing will be called for and the return address will have to be upside down, but it should go in one pass.

As for the rest, I agree.