Let me get this straight. You figured you'd fly into a busy international airport with a airsoft gun in your luggage and copy of a memo.
You thought that would satisfy the front line staff that it complies with the regs and that your piece of paper is correct? The answer of course is they won't. They'll seize it and let it get sorted out at a later date.
I guess you can hope that when the do test it they'll do it on a warm day, have a fresh tank of propane and the just right weight of BBs.
Good luck. Hope the hassle is worth what you saved vs. buying in country.
Airsoft, where nothing is hurt but feelings.