I managed to get through to B&You on the phone, by entering a failed order number (despite failed orders not usually giving an order number). They assured me that any physical French Visa card or MasterCard (not an e-Carte Bleue for example) should work. Thanks to VladS, I am therefore planning to buy a
Trans Cash prepaid Visa card for €9.90, which has the crucial French
BIN. Fortunately I now have a French address at which I can receive post (but addressed only in my name), so I can make sure the Visa card works before paying for the mail forwarding service for everyone else's SIM cards. The disadvantage of the Trans Cash card is that they
charge 3% for loading money on to it, but the advantage is that it's reloadable unlike La Poste's Carte Cadeau.
If you would like a B&You SIM card and haven't yet PM'd me with your e-mail address, please do so now.