What cheaper alternatives exist to transport luggage in Europe?

Essentially, you're after private shipping companies, I believe.

When I moved from the UK to Canada, I used Excess-Baggage.com to ship a couple of boxes over here. They ship all over the world, including from most major places in Europe.

I checked, and they do indeed service Portugal and the Netherlands.

It took its sweet time to arrive here, but the service was excellent. Note that there are some things you can't ship (stuff with batteries, for example), but almost everything I had was able to be packed and sent.

I usually put all the heaviest things (ie. books) in my carry-on and then get the biggest suitcase and stuff the rest in there, or 2 suitcases and pay the extra fee. This is possible if your things fit into the suitcase and are not strange sizes.

It is difficult to move that size luggage at a good price since it is too small for putting on a boat or truck but too big for a normal luggae on a plane. I also usually leave some stuff behind and label the boxes with 1,2,3 based on priority of bringing it to the new place, and when friends and relatives visit they pick up the next box.

I have used Seven Seas Worldwide in the past and they're really good and reliable. They offer door-to-door service.

They used to have this service where you start with one box which can hold up to 30Kg for 120 pounds and the next boxes is just 30 pounds per box. But not sure if they still have that service.

I'm not sure what you thing how much is expensive. I used to live in London and I shipped all my stuff in my apartment (books, clothes and other stuff) around 5 boxes back to Thailand. They arrived in Thailand in 30 days in front of my door with around 200 pounds.

And if you ship things across continent please check the content as you might be liable to pay some taxes.

I just got a tip from a friend that used Europarcel.

It`s not a traveller oriented service but it seems to offer good deals in some cases.

When I moved from Germany to Italy, I sent a parcel with household items using https://gls-group.eu/. Their prices were reasonable (around 20-25E for a parcel up to 40kg).

I experienced that shipping from Germany to other European countries is reasonably cheap (for example to Spain: 29€ for up to 25 kg with Hermes; 40€ for about 32 kg with DHL). The other way around options may be harder to find. I couldn't find any cheap carrier from Spain. However, Portugal > Germany is cheap with Chronopost (27€ for about 30 kg).