Yes, it can be done. Others have explained the process for sending your items.
However, understand that Portugal treats this like an international export shipment. So it goes SOUTH from Porto to Lisbon. Then it travels like an export shipment to Madrid, in the foreign country of Spain. From there, the Spanish Correos treats this like an international importation from the foreign country of Portugal. From Madrid, your shipment goes into the Spanish national postal distribution system. Your item will eventually show up at Ivar's.
In Portugal, shipping between two Portuguese places is simple, cheap and effective. Similarly, shipping between any two Spanish places, wholly within Spain, is also easy, cheap and quick.
The complications, and added time, come when you have to ship from one European country to another. Despite the EU being in place for some 25 years, and the Customs union longer than that IIRC, most all EU countries still maintain stovepiped, separate postal systems.
I explain this so you will understand that parcels rarely go straight line, as one would logically think.
If you allow enough time, you will be fine. It takes about 10 days to walk from Porto to Santiago. Ordinarily, that should be enough time. Ivar usually sends you an email when he gets the package if you put your email address on the label or parcel.
Hope this helps.