Honestly I don't know. They often find a reason to block it. When it gets blocked it is always for the same reason. They need proof that you paid for the goods and confirmation that it is for "personal use". I find this really stupid as it is obvious that it is for personal use. If I was going to sell the goods I wouldn't order the goods using my own name but them name of a store and I wouldn't order just 1 item but a set of 20 (just an example). And sure I had to pay for it. Since when it ordering stuff free? Would they send something over if you didn't pay? People at customs are stupid.
On top of that they always charge 21% VAT on the goods they block. This is also the case if the descriptions says :"DVD and Books". You're left with all the trouble because they are too lazy to make a difference in the value of the books and the DVDs. It makes a huge difference considering books are charged with only 6% VAT.
When they check your package there's also the problem that they keep your invoice. Usually you get 5 pages of documents with the package iirc. 1 page of those pages is the invoice and 2 others are the customs declaration documents. Well instead of using the declaration document they use they invoice itself. Not that big of a deal you might think but it is to me as the details of your order are on the invoice and you don't get the documents back once they checked it. You get the customs declaration document back but that one only has a vague description and only mentions the value of the goods and nothing about shipping charges and other details like discounts. This is a big deal for me as I keep track of everything I spend money on and I keep all of my invoices printed out in binders. Not easy when you don't get the invoice back after your packages got checked out.
Sadly I'm getting used to this and I often have to call them at one time or another of the delivery. I even took the trouble of adding their mail adress in my contacts list...
The only service I can't complain about are those from courier services like DHL and FedEx. They always fill in the customs the way it should be but you'll always have to pay charges as they take care of all the paperwork themselves (not the case with regular postal services) The only little thing you need to ba carefull about is the currency conversion. They sometimes change that a bit and you can end up paying more or less depending on the value and goods.
As you can see I'm clearly unhappy about the customs in my country.