1. Download the CTT app for your smartphone.
2. Locate a CTT office near where you will be in Lisbon before you start.
3. Send your parcel to Santiago. They have boxes for sale. Recommend a partial small role of 3M shipping tape as they seem to have none for your use.
4. Pray...
I have sent things from Portugal to Ivar at Santiago several times. ALL the parcels sent from Lisbon arrived fairly fast. All the parcels sent from places to the north, like Coimbra or Porto, took relatively longer. This is only a problem if you get to Santiago from Porto faster than your parcel.
It happens. In 2017, a medium sized box sent from Porto the morning before I left, arrived five days after I departed Santiago for the US. That meant a transit time of 16 days to go the same distance that I WALKED in 10. I tracked it as it moved, and did not move, first all over Portugal, then eventually straight to Santiago from Madrid.
In my experience, a parcel sent from Lisbon takes about a week to travel to Santiago. Bear in mind that all EU countries still maintain separate, national post services. Like pipes on a organ, this means that your parcel is still treated as an international shipment in terms of the logistics process.
So, all parcels sent from anywhere in Portugal go first to an international export portal at Lisbon, then to the internal shipment in portal at Madrid, then they go into the Spanish distribution system. This explains why things from Lisbon arrive relatively fast and easy, whereas something sent from the north of Portugal, like from Porto, can take longer.
The best thing about the app is that, once you send your parcel, you will have a tracking number. You can use the app to track your parcel... DO IT.
Fred Smith (the fellow who invented FedEx in the 1970s) could help them a lot. But, I think the national governments across Europe are simply protecting hundreds of thousands of civil service jobs. It does make logical sense... unless you are trying to get a package from point A to point B.
This said, within each country, Portugal or Spain, the postal services are a wonder. Services are inexpensive fast and efficient.
Also, unless his practice has changed, Ivar will usually send you a quick e-mail note to tell you he received your parcel.
Hope this helps.