Originally Posted by
westi2002
I booked the cheapest option once, since the text clearly said "Flying Blue members gain XP."
I'm still corresponding with FB helpdesk to get that 1 XP

So as some of us here have deducted, Reforestation gives 1 XP per 2000 miles or 10€ spent, rounded down.
1800 miles or 9,60 euro gives nothing, 2100 miles or 10,40 euro gives 1XP. (Miles/Eur amounts are fictitious)
in practice this first option hardly ever gives anything.
for SAF; it works the other way, it's 1 XP per 2000 miles or 10€, rounded up. So spending 3 euro on SAF, or 600 miles, will give 1 XP.
but good luck with FB CS.