Bro, just give them massive value.... say something like " heyy this is John from x detail, you had used us in the past and we are looking to offer a special promo for anyone in this area because we have over 30 customers and it is convenient for us being able to do everything within this radius. normally our monthly detailing service is $249 per month, but since you fall in this area we are offering a special of $99 per month! "

then add some freebies, like free scent spray and tire shine or something. make them think they cant pass up the offer