The other day I had to buy a FR to GER trip on KLM.fr because it wasn't available on AF.fr. As I wanted to combine gift card plus voucher payment which were not accepted on KLM website, I've called the PSL who then told me those voucher and gift card can only be redeemed on AF.fr - fair enough! I asked her if she could get a price through the phone for that specific itinerary so I could use them - indeed the fare difference was a bit more than 200€ on top of 60€ booking fee (that couldn't be waived even though the fare wasn't visible online)!
At the end, the KLM website fare won, booked/paid on the phone - without the fee of course - starting with 057 but unable to use these vouchers!
Strange that very simple itinerary couldn't (can't) be seen on AF website and/plus was priced much much higher than KL.