Seriously, think about it: here we are, the customer, dishing over our good money, for someone else's stuff - and it's us, the client; cash supplier; giver of coin - who says thank you.
Frankly, it just seems a little out of whack.
And too often, for my money.
If you think about it, shouldn't it go the other way? Shouldn't the person who receives the money - even if it's for awesome goods/service - be the one who says thank you, and us, the recipient of their goods/service, be the ones to say your welcome.
After all, we paid them; who should be thanking who?
Now granted, it certainly is a nice touch for the customer to say thank you, especially if the goods/service was exceptional. Be that as it may - the supplier of the goods/service should, at the very least, say thank you in return; better yet, get their first.
Can I get a woot woot?!
At a minimum - let's meet each other half way.
In other words, if the customer says thank you, then the supplier says thank you back. On the spot. No exception. And because we are nice people with good manners, if the supplier says thank you first (as they should), and they did a good job, then a nice customer should say thank you back.
Sounds like a fair and reasonable trade, yes?
Thank you = Thank you is Good (and Nice ;).