This was a knock and talk, not a controlled delivery. If it was a controlled delivery, they would have had a package (which you didn't mention them having), plus a warrant, which they could/would use to enter the house. They left because they didn't have a warrant. That means they really didn't have any evidence on you and by denying it you stood a good chance of not getting prosecuted. I think they do knock and talks because half the people get scared and admit it. It's a good way of nailing people when they have no other evidence on them, but what they really want is the sender. In this case, that's not possible. You're right though, it's a small amount. If your state has drug court / drug diversion, you'll probably get that or probation instead of jail time.