If the bill says gratuity added I consider that when figuring the tip. If it says service charge I tip as I normally would and I too tip 15% for service 20% for good service and more as warranted for exceptional service. For driving charters whether airport or excursion I tip the same. I round up in route taxis because it's quick and easy. For house keeping I leave $5US a day and make a point to get to know our house keepers and add a little more on the last day. I tip bellmen and baggage handlers $1US per bag minimum. And I always make it a point to get to know the security guys and put a little in their hand as well. It's always worked for me. Bless...
Ed