In the Envelopes dialog, click the Add to Document button. Then use the
Copies setting in the Print dialog, and put 0 in the Pages box (the envelope
is inserted as page 0). After you've printed, if you don't want to save the
letter with the envelope, click the Undo button to get rid of it.
Alternatively, make a template for envelopes and use that to create
documents that contain only the envelope, and print that normally while
using the Copies setting.