Written with people moving from Turkey in mind. The Portuguese rules still apply more broadly.
Zapping: Pay-As-You-Go
If you don't travel enough to justify a monthly pass, the "Zapping" method is the cheapest option. You load money (min €3) onto a reusable Navegante card, and each trip deducts a discounted fare.
A single Metro ticket costs €1.80, but with Zapping, it drops to around €1.61. It works across different operators (Metro, Bus, Train, Ferry), saving you from buying different tickets for each mode of transport.