AA & Delta are the two airlines which do this. Everyone else has to pay the $31 fee in cash before they fly.
P.S. Domestic Peruvian airfares vary widely. I saw $110-550 for LIM-CUZ (to see Machu Picchu). Here's my FriendFeed notes on different LIM-CUZ airfares.
http://ff.im/eDeFK
I ended up buying $120+tax=$145.18 for LIM-CUZ 2/10-2/17 from TACA on their website. Missed out on the $110-120 fare by not having my travel dates when talking to an Arequipa, Peru travel agent. Via phone they wouldn't give me the same fare.
...and there is a 10.18 soles ($3.57) domestic departure tax each way (payable in cash).
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Note: Taxi fare from LIM to Barranco should be 45 soles (no tip), but I paid 55 soles + 10 soles tip....before I asked some local people what the prices were. Taxi from Arequipa hostel to AQP was 15 soles (no tip). The prices in my guide books were low. For safety reasons, I didn't try the really cheap methods of travel. Too many warnings about crime in Peru. I had a great 7-day trip to Lima, Arequipa, & Colca Canyon. Looking forward to my next trip. Thanks OP for a great fare!
Some decent prices (Tourist areas will try to charge 2X-4X these prices), but this
Cafe in Arequipa had great ceviche for 15 soles ($5.26).
Buffets 10 soles (20 soles - tourist stuff or fancier) ($3.50-7.00).
Amazing homemade lasagna made fresh 7 soles ($2.46).
Combo dishes - chicken, hamburger, fries, drink, etc... 4-6 soles ($1.50-2.50).
Hamburgers are 3-5 soles ($1-2).
Street vendors sell chicken & beef shish-kabobs for 1 soles ($0.36).
Ice cream is not great. Stick to Argentina for ice cream, or just get the cheap 0.50-1.50 soles stuff.
Frozen juice bar on street 0.60 soles ($0.21).
500 ml Water 1 soles ($0.36). 0.60-0.80 soles in big grocery stores ($0.21-0.28).
500 ml Coke 1.50 soles ($0.54).
Hostels are $3-10. Over $5 is very pricey!
Awesome hot springs in Chivay 10 soles ($3.51).
Casinos have the best exchange rates (2.89-2.90 best I saw), 2.85 was the best saw I outside casinos in a competitive area in Arequipa (2.85 buy, 2.863 sell).