Purchasing your plane ticket gets the biggest traveling expense out of the way, but that total doesn't include anything you'd want to tack on to make your trip a happier experience. The Baltimore Sun gathered fee facts from domestic airlines and broke down charges for everything from beverages to pets [1].

• Beverages: Monster Energy drinks - $3 (Southwest Airlines); VitaminWater drinks - free (AirTran); $3 (Delta Air Lines); Beer, wine and alcoholic cocktails - $4 (Southwest Airlines); $5 (Delta, United, US Airways, American Airlines) $6 (AirTran Airways)
• Meals: Snack boxes and sandwiches range from $3-$8 (Delta, United, US Airways, American); Continental Airlines is the sole major U.S. carrier to still offer complimentary snacks (2+ hour flights) and meals (3+ hour flights); No sandwiches or meals available on Southwest or AirTran
• Checking Bags: Second bag - $10 (AirTran); $25 (American Airlines, Continental, Delta, Northwest, United and US Airways); Southwest charges starting with third bag - $25
• Seat Assignments: Exit row extra legroom - $20 (AirTran); $5 to $35 (Northwest Airlines); Window, aisle seats in front of plane - $5 to $30 (US Airways); Reserving seat number in advance online - $6 (AirTran); Business Select early boarding - $10 to $30 more than highest fare (Southwest)
Find out how much airlines are charging for other privileges when you .
• Unaccompanied children (nonstop flights): $100 (Delta, US Airways); $99 (United); $75 (American, Continental); $75-$120 (Northwest); $39 (AirTran); free (Southwest)
• Curbside Check-in: $3 per bag (Delta); $2 per bag (Delta, United, US Airways, American, Northwest); Free (Southwest and AirTran)
• Headphones: $5 (US Airways); Free on United, AirTran; No in-flight entertainment (Southwest)
• Pets: $65 one-way for small pets in a carrier that fits under the seat (AirTran); $100 for small pets in cabin to North American locations (Delta, US Airways and United); $95 (Continental); $80 (American, Northwest); Southwest doesn't allow pets to be brought on-board or stored as cargo

Source [2]