you should not have a problem with a flash cart(i never i did).
so far i have bought a r4 original, a r4 1:1 clone(use's the same firmware as the real r4), a acekard 2i, acekard 2(sold to my cousin), a m3ds real(when they first came out), a r4igolc(.cn version) a gba firelink(or was it a flash link)
so so far 7 flash carts for gba/nds+ EMS USB, Everdrive, Everdriver MD(Everdrive MD clone and my e3 flasher
i buy seed all the time for my dad from UK and they never check them, hell i know someone that bought pot seed only and it came fine(not opened)
form my experience customs here in portugal only look at the size of the package, so if its a big box( 360 or ps3 sized box) they will open it regardless of how much you paid, if its from europe
so you might want to buy from
discoazul.pt they have the dstwo for altogether 35,43€(for me and i live in açores) i bought my swapmagic 3.5 + the magic keys 4 for my v18 ps2 from them took abot 4 days(shipped from spain to azores)