What a bunch of turkeys. There must be better use of resources than to open a package with a t-shirt.
Do you think it was, possibly, just a random check. If not, something about the package alerted them, i.e if shipping document says t-shirt and they feel something hard inside.
Could have been other suspicious items in the shipping/mail container and they checked everything.
Good deal on the guarantee with stealth shipping.
If you think it will happen in the future try a new route via a Canadian seed bank.