I don't believe a "custom-leather-maker-to-customer" relationship is quite the same as a JC Penny, or Cheap Tickets......or any other nationally known company you order online from.....
They took interest in your work in the first place, which is why you did something for them.
I think it's a good idea to shoot them a "howdy" note now and then with maybe a picture of your latest project they may like to see, or maybe just directing them to your web site.
A newsletter format is a good idea too....it's called marketing, any business should have a marketing program.