Is adding a 1% commission on every ticket sold to the customer, and increasing it to 1% monthly when paid in installments, permissible or forbidden?

If the company sells airline tickets and buys them from a travel agency, there is no objection to adding a commission or selling them in installments with an increase in price, provided the contract is based on a specified price. However, if the company does not operate in booking airline tickets and merely performs this as a service for its customers, then it is not permissible for it to increase the price without the customer's knowledge and consent.