Hi.
It's not the most elegant nor refined scenario, but you can print envelopes with TextEdit as follows:
Open a new TextEdit document (File/New), then go to File/Page Setup, and in the Paper Size drop-down menu, select the envelope size you want to print to, and the landscape orientation (wether the size of envelope is on the list or not, very much depends on the drivers installed for your printer).
Then go to Format, and select Wrap to Page, this will show you in plain white the envelope surface, and will slightly gray out the non printable area, also will display a box in which it "suggests" to write in.
You can save this and use it as a sort of template.
Good Luck.