Planet13 - 7:15 am on Jan 4, 2012 (gmt 0)
Maybe I am being too sensitive.
Yeah, because really, at the end of the day, what you actually want is their business (and their assistance in helping to promote your business), not their thanks.
I've messed up orders for customers in the past, and they were pretty irate. By apologizing and telling them the truth (even when I wanted to yell at them), and by giving them a discount coupon for a future order, I have turned some of them into repeat customers. It hasn't been killer for me, but every bit helps.
So what I think your client needs to do is just somehow make every contact an opportunity to market his products / promote his brand, and not take it personally when their isn't a thank you email.
In the real estate world, they say, ABP: Always Be Prospecting, and that is what I think you have to do when people send you an email. Not necessarily "push" them to buy from you, but at least mention what you got and why it is so special.
Also, I like wheel's suggestion of getting them to call (Note: Unlike most mortals, wheel has the gift of gab. If you have no phone skills, you might want to write a script of some sort before you invite people to call you so you at least have an idea of how to begin and end a conversation in a courteous manner).