We had to provide a Negative Covid Test and Certificate ... we had our done at a small testing lab on Ermou Street - cost 5 euros for an Antigen test.
We met a couple who thought all they had to do was show proof of vaccination - wrong !!. They were tested at the port and charged 60 euros each for a simple antigen test.
We had four days in Athens and were worried we may test positive ..though we had been ever so careful.
We tested ourselves pre flight after flight, first day in Athens ... all negative and after our Official Test ..negative we had our certificate.
It is stressful ...so we are pleased we don't have to go through all this for boarding in Lisbon.