These "proof of passage" receipts. I may need to do these on my upcoming ride. Do I just go into a shop or garage and buy a Mars bar or something, and keep the receipt, is that all that's required? Then what? Do I hand the receipts over with my card at the end? Or do I have to write a control number or something mysterious on the receipt?
Yup, that's all. Have a look at the receipt before you leave the shop, make sure location is printed on it and the time is legible and correct. You can then write the time on the card, and the location on the receipt if it's not clear. I usually stuff them all in the ziplock bag with the card, and the controller helps sort them out at the end. If the cards are being sent to france for homogulation (not usual except in a PBP year) they'll stick/staple them to the cards for you.
Cash points are useful as their clocks are accurate, and if you aren't hungry/thirsty they save you spending money.
Edit - it's not meant to be hard, just a way of proving you were there. I'm pretty sure if something went wrong with your receipts, and you could take out garmin/phone and show the GPS track, a controller would be happy to accept it. On my first 200km audax I realised that the receipt from the cafe had the wrong time, so went into the coop, and their clock was wrong too. Cafe owner initialed the receipt, and org had no problem with it.