The post within Spain (Correos) is very reasonable. The post in Portugal (CTT) is nearly as inexpensive. It is FAST and reliable. In April 2015, I shipped several packages from Porto and elsewhere in Portugal to Ivar in Santiago. It all went smoothly and without drama.
Go to a CTT Portugal post office in Porto and ask to mail your bag to Santiago. They will tell you if you need to box it, wrap it, or whatever. Also, I bring a small, partially used roll of 3M shipping tape that I buy in the supermarket to help seal the post office box. They usually need a little more tape.
My personal choice is to use a soft duffel bag to get things to my starting point, then place everything in post-office supplied (or purchased) boxes, along with the folded duffle bag. Everything winds up in Santiago, and I have my duffel bag too. I do this to avoid the postal official telling me they do not ship luggage.
ALWAYS have a Plan B!
I hope this helps.