How to print writer form doc with letter + envelope

I have a writer doc form letter with an included form envelope. How do I print this? What about a batch of prints from a spreadsheet? I need letter+envelope files to print custom, per-addressee, correspondence.

I know how to do form letters.
I know how to do form envelopes.
My effort might be as much a “printing/printer” issue as a writer issue.

Thanks in advance,

I just tried a simple print with a sample document and did Insert > Envelope. I clicked print, disabled double-sided then OK. My printer printed the document on A4 then selected the envelope tray to print the envelope. In Windows Printing Preferences in the printer driver under Paper source I have Auto Select chosen. I suspect a mail merge to print would select the correct trays for each because it seems to be the printer driver managing the paper source.

My other printer cannot do this trick because it has only one print tray.

The letter is a form letter, as is the associated envelope…fields from a registered spreadsheet. The printer needs a tray change by hand to swap from letter paper to #10 envelope.

The easiest way is to use a window envelope. This is what window envelopes were developed for.

The other way would be two mail merge print runs, one for letter and one for envelopes. You would need a separate form for the envelopes. The first problem with this is making sure you match the correct letter to each envelope, even when printed in same order and same selection; the second, lesser problem is that you might be limited to a small number of envelopes in the print tray or the output tray. I find envelopes produce a high number of paper handling errors compared to sheet paper