Does anyone know the value limits for importing goods into Portugal from outwith the EU without incurring import duty? I once bought something off eBay from China for a couple of hundred pounds, and had to pay about 85 Euros duty at the post office before they'd hand it over! Not quite such a bargain after all :-(

Not sure about the limits, but when based in Portugal and whenever we bought stuff off eBay outside the EU we would have it shipped to the UK first... second delivery address was family in the UK. They would then pay any taxes and ship in onwards. Another way we used to get around it was to have items shipped to Algarve Freight depot in the UK. They will pay any import taxes due and ship it onwards to Portugal. You pay taxes with the delivery charge on collection. We did that a few times also. It would always work out far cheaper and quicker than trying to deal with Portuguese customs.

Trying to import anything into Portugal from overseas is a total Rip Off and Alfandiga (spelling?) damn well know it!

The only free trade within Europe is between EU members imports from anywhere else can attract tax. Portugal is not doing anything different to the UK.
Many people in the UK get a shock when buying off Ebay or Amazon when they get a bill because the product has come from outside of the EU.
That is why you should carefully check where you are buying from.

I'm not sure what the limit is; usually it's a few hundred euros, + a certain amount worth of cigarettes and a certain amount for booze.
But that only applies when you travel.
For mail order items, you [theoretically] have to pay duty from the first euro.
It practice, they let a lot slide through, especially smaller parcels. But I've learned to order only from within the EU if at all possible.
Things from outside the EU can get stuck in the customs queue for weeks or months. Then you have to pay duty + IVA on the item + duty + shipping.
It just takes all the fun away.

It's not worth sending anything to Portugal from outside the EU. It sits forever in customs and they make up the duty and iva as they feel fit. If you can have it send to another EU country and pay the extra postage and get it before you die.
I have a camera sitting in customs since March, emailed them and only get automated answers.
As far as the post is concerned, don't send or receive anything in August, you wait forever.