As others have said it's endemic nowadays and seemingly across all business types, but retail is obviously the most in your face.
I too just politely decline as I've been on the other side too.
I've just left a parts position at a volume main dealer site, and we actually had a list of associated products, (slightly differs to direct upselling IMO, but the end aim is the same), on the wall for each adviser to see.
So if you price up a radiator you could be expected to be offered anti-freeze, order an oil filter, "do you need any oil?", brake pads, "we've got an offer on brake cleaner/fluid" etc, etc, etc.
It has it's place, but in general I've always had the impression people think you're simply trying to pump them for more cash, well you are aren't you, but people don't want to feel that's what's happening.
Slightly OT, but the one that grinds my gears these days is in the supermarket, "Do you want any help with your packing?", and they're not even trying to sell me more of anything. They're just trying to reinforce a positive customer experience, but it really makes my teeth itch
I'm still polite though as the poor soul working the checkout is only doing what they're told by the big cheese.