If you’re using a good print shop, then you shouldn’t need to worry about this.
You simply send them the Bleed Ready PDF file along with your Prospect List and they’ll print them out, cut them to size (using a commercial paper cutter), fold them & stuff them into envelopes (using a commercial paper folder/envelope stuffer), mail-merge the labels from your prospect list and get them in the post for you.
It’s quite an amazing process to watch and one that you definitely should not think about doing yourself unless you’re doing BIG volume or a lot of internal testing 