Custom charges?

Anything being imported into the EU is liable to be charged duties or VAT, or just a randomly charged customs handling fee, on arrival. In my experience, for small items, whether or not it gets assessed in customs seems to be pretty random. I get plenty of stuff from the U.S. Sometimes I get charged, others not. There doesn't appear to be any rational explanation. Larger packages (no matter what the value) are more likely to be stopped and controlled.