Tickets Bolivia charges a fee that is calculated for each transaction, depending on the trip, normally this is defined by the distance of the trip. This fee is described before the payment is made, so the passenger knows exactly how much the bus cost is and how much the fee of Tickets Bolivia is.
You should take into consideration that there are also some other costs to consider, and which is described as well in the last step of the buying process. Costs like IVA (tax) and the secure payment gateway, mainly handled by Stripe and PayPal.
Most of the bus terminals in Bolivia charges a "terminal tax" that is not included in the e-ticket from Tickets Bolivia. This fee varies between 0.30$us to 0.40$us, and passengers must buy it inside the Terminal.