Tokuisaki (得意先): a customer who always buys from one, or a partner one trades with as a matter of course. A dictionary gives torihikisaki, trading partner, as its synonym.
Kongo tomo yoroshiku onegai itashimasu (今後ともよろしくお願いいたします): we ask for your continued favor from now on. A business magazine explains that the phrase expresses a relationship that will continue over a long span.