Well, first of all realize if you dealing with a retail establishment Jewelry stores have the highest markup of just about any area. When they have a 1/2 off sale they are still making too much.

Most sales people in Jewelry stores earn commission so it depends is this a salesperson, the owner or an actual designer. If he's a sales person and you think he actually went the extra mile for you, though not customary you could give him a tip. If its the owner the answer is no and if its a designer they usually make a handsome salary so I wouldn't there either.