I love the fact that my newspaper delivery guy, who delivers 352 days a year in all kinds of weather (his ten-year-old sedan apparently can handle heavy snow at 5am better than the post office trucks can handle even light snow at 4pm) sends me a holiday card with his name and address at this time of year. This is my reminder to send him his annual gratuity, which I appreciate. I never see him and my billing is through the newspapers he delivers.
He always sends a nice “thank you” note in response to my gratuity.
There’s nothing wrong with you reminding your clients how much you’ve accomplished together over the last year. The point wouldn’t be a gratuity, it would be expansion business. And you don’t have to drive in the snow or get up at 4am.