It's completely random. Sometimes things will go through Customs unchecked, sometimes they are caught at Customs but the Post Office doesn't do it's job properly. It also seems to be random as to whether the PO tries to collect the charge at delivery - which is what they claim they now do (but the posties don't notice the massive CUSTOMS CHARGES DUE notes, don't know what they are, what to do) or whether they send you one of those letters saying you have to go to some obscure depot to pay up and collect the package (this last leaving you wondering why you pay postage in the first place and what exactly the "Handling fee" pays for). It makes good sense to collect on delivery (I did once, and they take credit carRAB with a posh handheld that takes signatures). I queried it once cos I had visions of the Customs or PO paying me a visit, but it's the postie's responsibility to collect the fees. And they frequently don't. Theoretically they don't hand it over until they've recovered the fees, but they don't seem to track anything properly, no wonder the GPO makes huge losses!
It's bizarre. I had to go and collect and pay fees , paid on delivery, had parcels with fees due but the postie wouldn'ttake them, and quite a few that slipped through Customs altogether..
Even the amounts don't count. I had to collect and pay on a tiny package with a few quid due, while another package worth