Mailing done by a bulk mailer, using a second set of mail assemblers "Work from home, earn $$$ for only a few minutes a day", who spend the entire day simply collating the printed mailers and stuffing envelopes. Paid around 3c per filled envelope. Then the filled envelopes are sent using a bulk mailer, so the actual price paid for postage is likely less than the franked amount.
As to the personalisation, you will find the non black text is preprinted, with a lot of the boilerplate print as well, only the sections with a personalised part will be laser printed. Then the full sheet of paper is split on the perforations ( so you can print the entire thing in a large commercial laser, likely using a continuous sheet of prepunched and roll printed paper) and stuffed into the envelopes.
The return address is the local associate who gets the mail, and then fills in the results for the second round of mailing. Then they will do it again with a more directed approach so as to get a small donation out of you ( then a larger one of course, gotta pay for the drop office and the local guys) to pay for this operation.
As to how they got your address, it was done with a web crawler looking for physical addresses off websites and social media.