You applied one month ago and already got an invitation? How tf does their application-reviewing system work when people sometimes wait for half a year?
I found Skytest to be very useful, it was almost 100 % the same stuff. Best of luck! Also may I ask where you're from? Seems like they prefer Swedes for their Stockholm Base and stuff like that. If that's not the reason you got invited I have no idea how their system works.