A method for charging fees for location usages by onboard units (21, 24) in a road toll system by means of a separate payment transaction system, wherein the road toll system has a toll server, at least one toll terminal and at least one sub-system (16, 17) for determining the location usages, and the payment transaction system has a transaction server and at least one transaction terminal for payment cards, wherein every transaction terminal is assigned to one or more toll terminals, said method comprising: inputting an identification of an on-board unit (21, 24) into the toll terminal; generating a unique transaction identification corresponding to the on-board unit identification in the toll terminal, wherein the on-board unit identification is not derivable from solely knowing the transaction identification; transmitting the transaction identification from the toll terminal to the transaction server via a first data connection therebetween; transmitting the transaction identification from the transaction server to the transaction terminal via a second data connection therebetween; inputting a card identification (CC#) of a payment card into the transaction terminal and transmitting the card identification (CC#) and the transaction identification from the transaction terminal to the transaction server via the second data connection; wherein a transaction value (T$) is transmitted either beforehand from the toll terminal or later from the transaction terminal to the transaction server; transmitting a substitute identification (AL#), generated in the transaction server and unambiguously associated with the card identification (CC#), and the transaction identification from the transaction server to the toll terminal via the first data connection; retrieving, in the toll terminal, the on-board unit identification associated with the transaction identification and transmitting the on-board unit identification and the substitute identification (AL#) from the toll terminal to the toll server; and using the substitute identification (AL#) in a communication via a third data connection between the toll server and the transaction server for charging a fee for a location usage determined by the sub-system (16, 17) under the respective on-board unit identification. inputting an identification of an on-board unit (21, 24) into the toll terminal; generating a unique transaction identification corresponding to the on-board unit identification in the toll terminal, wherein the on-board unit identification is not derivable from solely knowing the transaction identification; transmitting the transaction identification from the toll terminal to the transaction server via a first data connection therebetween; transmitting the transaction identification from the transaction server to the transaction terminal via a second data connection therebetween; inputting a card identification (CC#) of a payment card into the transaction terminal and transmitting the card identification (CC#) and the transaction identification from the transaction terminal to the transaction server via the second data connection; wherein a transaction value (T$) is transmitted either beforehand from the toll terminal or later from the transaction terminal to the transaction server; transmitting a substitute identification (AL#), generated in the transaction server and unambiguously associated with the card identification (CC#), and the transaction identification from the transaction server to the toll terminal via the first data connection; retrieving, in the toll terminal, the on-board unit identification associated with the transaction identification and transmitting the on-board unit identification and the substitute identification (AL#) from the toll terminal to the toll server; and using the substitute identification (AL#) in a communication via a third data connection between the toll server and the transaction server for charging a fee for a location usage determined by the sub-system (16, 17) under the respective on-board unit identification.