It's is all about making sure that relevant users click on your ad
A relevant user is one that is likely to perform the action you wish them to on your site. To make sure you are getting the right kind of clicks, you should strive to set the right expectations in your ad. Give your ad the same look and feel as your landing page and try not to leave important information like shipping costs out. You want there to be as few surprises as possible when the user lands on your page. It might also be a good idea to state the price of your product or service in the ad, this will discourage users who are not prepared to pay what you are asking or who are looking for free offers.