Kerri Sox - Before I cruise - I go online and purchase the drink coupons. One you have the coupon...there is no more charge. Just give them the coupon for a drink...no gratuity charge at that time or when you purchase them.
That being said, don't try and use them for the nice "Drink of the Day" and expect to get a drink in a large Carnival Cup. And don't try to get a top shelf mixed drink (say....Grey Goose Martini straight up, dirty with two olives
But for the generic mixed drink in a small glass, they are actually $$5.00 with no tip.
Hope this helps