Had an order seized once. Made the mistake of ordering from a non-EU source (living in mainland Europe), which increases the probability of getting your shit seized manifold I reckon (especially when your source ships from a "high risk country"). Didn't respond to the seizure notice, as recommended by some fellas on a national board I browsed during the time. Got a letter from LE a few weeks later, gave me the choice of either paying a moderate fine and have all charges dropped, or having an investigation launched. Decided to do the former and was let off the hook.
No idea what would have happened if I just denied everything, since it was a very small order. No idea what kinda investigation they would have launched for some guy ordering an absolutely miniscule amount of PEDs. In retrospect I think that's their standard procedure, and if I had just ignored it, nothing would have happened, but I guess I'll never know.
Been ordering from an EU source ever since without problems.