Two things. You don't have to use a credit card. Seed botique allows you to send in your payment by mail, although if the cash can be seen through the envelope then it will most likely never reach its destination.
And numero dos, yes, you are breaking a law.
But they can't do anything to you if they find an envelope with a couple seeds in it addressed to your house. Anyone could of sent them, for any reason. And noones going to care if ten seeds are found in an envelope. 500 seeds, something might reach the Higher-ups, but it still won't be much of an issue. They'll simply take the seeds, and leave a little note saying the contents were taken by customs.

Credit card companies don't verify what you bought or where you shipped it to.

Oh, and it will most likely pass customs. If I remember correctly, seedbotique has a very... unique .. packaging system.