FWIW, I abandoned the quite efficient "specialized printing programs for envelopes" which I had used for at least 15 of the 27 years I've had my treasured Macs . . . because Pages (and its Templates) came along. Does everyone who needs to have access to printing your "form onto an envelope to use for clients" have access to Pages?
The initial design of a personalized template may be complex, but once you start with a page layout blank, all the bells-and-whistles in Pages makes almost anything possible – down to fractions of an inch. It's typical "Mac": everything you want to accomplish is probably "in there" you just have to find it in sometimes collapsed/buried menus. For example: the Small Caps font adjustment is @ Pages Menu––>FORMAT––>FONT––>CAPITALIZATION––>SMALL CAPS. The set-up effort is worth the future usability, IMHO.
I have temps for envelopes from Monarch to Nr. 10 to catalogue sizes w/ and w/o return addresses, logos etc. etc., mail labels etc. I can even print personalized checks (wallet/ledger/etc) from my Pages Templates.