1. A flight plan is NEVER connected to a French Customs PPR or PNR.
2. Customs airfield : inbound AND outbound : still a customs airfield for the return flight (with sufficient notice).
3. Retain a copy of your Customs PPR, if challenged by the 'douaniers'.
If the French Customs officers turn up (in fact rarely), and if these rules are not complied, the pilot is subject to a hefty 'fine', say about £500. No explanation or discussion is possible with French 'douaniers', people are considered as guilty, unless they prove they have complied with rules and prove their 'bonne foi'.