The Directchat fee is unique, its not a set "5%" for example that Directcam has.
Adultworks help article is here.
The key part is, "has an imposed charge of 0.15 credits per minute" which essentially means if you charge 1 credit, the fee will be 15%. If you charge 2 credits the fee will be 7.5%.
The more you charge the less the facility fee is. If you charged 0.30 credits per minute for example, the fee of 0.15 per minute would be 50%.