Here is a way to print envelopes using LO Writer in a straightforward way without having to resort to workarounds or other page formatting tricks.
I had been struggling with getting envelopes to print properly for some time now and I was at the point where everything seemed to be working except that the output from the printer would be too far to the left, as if printing on standard 8.5"x11" paper even though the page format was set to #10 Envelope. Because the manual feed tray on most printers automatically centers the media along the paper path, I worked around this by leaving the feed tray guides set for 8.5"x11" paper and inserting the envelope against the guide on the left side. This produced the desired result, but it caused other problems. The first is that the printer (at least on my printer) went into cleaning mode immediately after having printed the envelope. And the second is that (again, on my printer) the printer kept trying to re-print the job because it thought the job had failed. Cancelling the job at the printer did not work, so the simplest thing to do was to just let it print again on an 8.5"x11" sheet. But now, I no longer have to go through any of that soft of nonsense, and neither will you.
I`m using Linux Mint 19.3, LibreOffice 18.104.22.168 and an HP MFP M479fdw, but given the nature of the fix, this should work with any OS, any version of LO Writer and just about any printer.
The solution lies in the printer’s tray configuration. Here are the steps I took to get things to work the way they should:
If you’re like most users, your printer’s paper drawer contains 8.5"x11" paper and the manual feed tray can be used for other media, such as envelopes. In your printer’s tray configuration menu, set your main tray (drawer) to 8.5"x11" (letter), and set the manual tray for ‘#10 Envelope’. You can leave the ‘Paper Type’ setting for both trays to ‘Any’.
Start up LO Writer, create a new document, go to ‘Format → Page Style’, and on the ‘Page’ tab under ‘Format:’ choose ‘#10 Envelope’ and under ‘Orientation:’ select ‘Landscape’. Once you click ‘OK’, you should see a blank page that has the shape of a #10 envelope.
You can now create your envelope by inserting text boxes (‘Insert → text Box’) where needed, filling them out and moving them around the page as needed. Personally, I create two text boxes - one for the return address and another one for the destination address. Alternately, you can just type in the return address at the upper right corner, hit ‘Enter’ a few times, move your left margin over to the right about 3 inches and entering in the destination address. Either way, your envelope will print correctly.
Once your envelope looks the way you want it to, slip a blank envelope (side edge first) into the manual feed tray of your printer and adjust the guides to the size of the envelope. This should place the envelope in the center of the paper path. Then just print the document as you normally would.
Because your page format is set to #10 Envelope, your printer should not use the main paper tray (which is set to 8.5"x11") and instead use the manual feed tray which is set to #10 Envelope. This setting will also cause the printer’s output to be centered, matching the position of the envelope. This works flawlessly with my setup and I certainly hope it does for you as well.