Yes, they are easy to put on... so bargain with them
Which cover you get depends on how much air flow and opening you need... in most cases, they were just used to let the heat out rarely for us, so no big deal if if flows 10% less air or doesn't let the cover open all the way ...
Show them the internet price and see what they say - for $50 I'd let them install them so I wouldn't have to get up there