I bought from them on monday and I'm within the U.K. They marked it shipped on Tuesday morning and it still hasn't arrived. I also bought some dmt that has arrived which was also shipped on tuesday. Is it possible that they're selective scamming or could it have got caught in the post?
"Should I be worried?"
Long story short: No. Unless you mean monday and tuesday of last week, in which case the answer is: Probably not.
I'd wait at least 7 business days before even beginning to worry about anything.