In Cyprus the rule is that if the declared value plus shipping is above 45 euro you pay taxes. (19% + 3.50 euro for custom services).
Once I had an exchange with Canada and because the shipping alone was 75 usd I had to pay another 20 euro for custom fees.
Another time, a swap from Taiwan, the declared value was 10usd and the shipping cost 6usd but again I had to pay custom fees (around 5 euro so I didn't bother asking why).
I always complete the customs declaration form, because the post office employees of my neighborhood instinct that it is mandatory for all non letter mail even inside the EU, even though the official post office website states otherwise.
Also I declare my packages as “Hobby material”. If the declared package is label as “Coins” and it comes outside the EU, they may send the coins to the department of antiquities to check the coins. That happened to me 3 times, even for coins coming from Spain. In that case the package may take 2-3 months to arrive.