No matter what my settings, the return address on the #10 envelope is either 1 1/2 inches from the top, or prints on the flap

envelope problem in Libre

You do not indicate your operating system, printer model, printer connection (USB/network), printer driver, or protocol (PostScript, PDF, etc.). It is likely that the information in either this question or this one will be of assistance. Both concern #10 envelope issues.