Not sure what your routing is but it is very simple to land at LFAT pay landing fee and kick on. Customs may or may not be there but you would have complied with the regs. You have entered Schengen so can go on anywhere you please.
On return fill in a Gen Dec 4 hrs before departure (if you are all UK/EU citizens)and email to
[email protected] or via an app like Sky demon.