I have ordered from them twice with good results.
Check you email again. They send you 2 emails: one to confirm processing and one to confirm shipping. They offer tracking through royal mail. (you can track them until they reach US, at which point they are just sent through U$PS)
It may be your package was confiscated by customs. If so you will probably be getting a letter stating so.
If this ends up being the case, I would keep the letter as proof that you didn't receive your seeds, because I believe Single Seed Centre offers guaranteed shipping.