Well mushroom grow kits are not illegeal so technically customs has no authority to do anything but once in a while you find a few dicks in customs.
If your product does get seized, you get a letter in the mail stating that they seized your package and they issue a warning. No leagal action can be taken the first time they seize a growkit because they are not illegal.
The letter will say something along the lines of "If you can prove this kit is for educational or labatory use then they will release the package".
Good luck with you Order, and happy trips!