Don't fly with Aires Airlines
If you happen to be neither Panamanian nor Colombian citizen and want to use their service between Panama City, Panama, and Cartagena, Colombia, be prepared to have an onward ticket. Aires is a small Colombian airline with mostly domestic service using small Dash 8 aircrafts. Their only two international destinations are Panama and Maracaibo in Venezuela, which was added recently.
Today we showed up on time to fly back to Panama City, our origin, and were refused boarding due to lack of an onward ticket. Arguing with their ground staff did not get us the boarding pass. They claimed each passenger who is not a legal resident of the destination country needs to have an onward ticket. According to them this were international law and each and every airline would work the same way.
So after a while we lost our patience and walked away to buy a one-way ticket from Copa Airlines, the largest Panamanian airline, without any questions asked. We got the boarding pass right away and everyone was very friendly and helpful.
Currently we are waiting here in the airport of Cartagena for the boarding process to commence. Apparently there is a free WiFi service, but I can't connect to it. So this post has to wait a few hours until we arrive in Panama City later today.