Hi Jen,
Heather from 6000+ km away in PEI.
Here are a couple things that happened to me that are easy to take care of beforehand...
I flew to Biarritz from England before my first walk along the Camino. Going through French Customs was easy. So easy that I wasn't even given a stamp in my passport. I never thought about it then but when I was about to board my flight to fly back to England, after I finished my walk, the Spanish Customs almost did not let me leave. Make sure you get all the proper stamps in your passport--your real passport, not your credential, the pilgrims passport. You should have no problems if you are flying into Paris with getting a stamp.
The next walk, I had arrived in Toronto and was walking towards Customs. Before I stood in the line up to make my declaration, I spoke to a Custom's agent. She asked how much I had spent while I was away. I didn't take meals, alberques--the Camino costs--into account, but the things I had brought back with me. For a seven week trip away, $400 was very low. She was ready to not let me go through until I explained what the Camino is.
The next walk I made sure all passport stamps were in place and that I kept in a small notebook reciepts for things I'd be taking back to Canada and (for my interest only) a daily tally of what I'd spent. Yeah, that last part lasted about 4-5 days. But when I was to go through Canadian Customs, I had the reciepts and the total of €€ spent and some info about the Camino. Even with this preparation, it was smooth sailing through. I think it does depend on who you get in Customs. And being friendly and polite can also make things a lot easier too. I would not for the world be a Customs agent--what a stressful job!
You'll have a great time; the Camino is a wonderful experience and the
Camino Frances is so well set up with amenities for pilgrims that you can focus on walking, eating and sleeping!
Buen Camino!
Heather