Dear Alberto,
You can follow below steps to achive the same.
1 You have to setup PPF framework.
A. You will be having output profile (which is for processing the actions in background) assigned to Air FWO document type.
B.Go to PPF customizing (Tcode - SPPFCADM) -> Select the Action profile -> Create Action definition ->choose the processing time as 'Processing when saving the document'-> processing type as 'Method call' -> Create the BADI implementation for definition EXEC_METHODCALL_PPF''. Here you can check if any filter before you trigger XML Message.
2.You have to create a custom Proxy and proxy class to generate the XML message and send to target system.
You will call this proxy class in BADI implementation EXEC_METHODCALL_PPF and build the XML message with required Charges data.
After all the setup is done, you can test and make sure you have XML message generated in SXI_MONITOR / SXMB_MONI t-code.
Let me know any challenges while doing it.
Thanks,
Bharath.