Flight costs have to be stated in the same currency as the ticket has been paid. In that case in a letter you address to the project officer just mention the reasons. These reasons have not to be put in the financial data. Check inputed costs in particular overhead that must be accepted by your contract. In regards to your view on the project give suggestions to your project officer for agreement.
Justifications have to be given for the advance.
After approval of final report and overall project status final payment will be done. In some rare cases it may be necessary to extend your contract without increase or modifications of the budget. You have to send a formal request to the Commission. By email answer using the reply fonction of your mailer with the content of the mail or answer addressed by your project officer. Financial services, control services, payment services and finally been paid to the bank account related to your project.