I can't help. I print labels from the Home Base and use my own solid, secure, opaque envelopes with my return address and logo.

I don't like to advertise to the post office or the carriers or anyone else that "There's a tax return and documents inside!"

Use the ProSeries option and off the slip sheet. Purchase the envelopes from Tenenz.  These are coded so you cannot see inside and on the outside of the first class envelope it just has "Important Information Enclosed".  I do not like to inform anyone handling the mail that there are tax documents enclosed.