Fees are automatically calculated and are based on a number of factors. The two main factors include both the amount of traffic there is on the network, as well as the size of the transaction in terms of inputs. 

Airbitz makes zero profit from these fees, with 100% of the fee going to the bitcoin miners. For a more in detailed explanation on how bitcoin mining fees work, please visit the following link:  https://blog.edgesecure.co/what-are-bitcoin-mining-fees-and-dust-323d5469c572