Checking a couple of pieces of luggage (so long as they weren't oversize or overweight) was for years a complimentary service for air travelers. That all changed in February 2008, when United and US Airways began charging $25 for your second checked bag. A few months later American raised the bar by instituting a $15 charge for the first piece of luggage. Now the going rate is $25 for the first bag and $35 for the second on most flights. The unsurprising result: more flyers began carrying on their luggage, leading to weary shoulders, cramped overhead compartments — and No. 17..