I have done this before with another business I owned previous to getting into lawn care.
Advantage: Rate much cheaper then 1st class. I think it was around 6 cents apiece when I did it last - probably 6 years ago.
Disadvantage: You have to buy permit. You have to "obtain" names for mailing. That means either creating your own database from somewhere (time consuming) or purchasing a commercial list (expensive). Then you have to have your envelopes (with your PERMIT # in place of where stamp would go) printed up. But the BIGGEST drawback was the SORTING. You have to sort them by ZIP. Each in their own little tub & wrapped in approve wraps. THen physically take them to MAIN post office (Downtown) to have them weighed in & checked in. Depending on how many other people were waiting this could take a few minutes to more than an hour.
In my old business we marketed to people thru-out the city. In lawn care you could just shot-gun a zip code in the area you want to hit. Maybe not as bad.
We also included a POSTAGE PAID REPLY CARD. These were not cheap! Again you had to have the PERMIT. Plus each one that came back cost 59 CENTS. Most returns were from @SSHOLES that thought it would be funny to send them back empty.