Looking at the Portuguese post office website (feb. 2017) using the "Encomendas" shipping rate. Now, I am not an expert in portuguese, but I think this is correct. These rates are: (Spain is in "Zone 1", the left column).
Remember that the largest box that the portuguese post office sell is very small. You might want to get a larger box before you go to the post office.
https://www.ctt.pt/correio-e-encomendas/enviar-correio/opcoes-de-envio/encomendas.html#panel2-1
You can find a post office here:
https://www.ctt.pt/feapl_2/app/open/stationSearch/stationSearch.jspx?lang=def
Rates from february 2017.
Remember that the largest box that the portuguese post office sell is very small. You might want to get a larger box before you go to the post office.
https://www.ctt.pt/correio-e-encomendas/enviar-correio/opcoes-de-envio/encomendas.html#panel2-1
You can find a post office here:
https://www.ctt.pt/feapl_2/app/open/stationSearch/stationSearch.jspx?lang=def
Rates from february 2017.