The fare when purchasing tickets directly on the bus is €2.50 for Day Services, and €3.00 for Night Services. Night Services are identified by the letter N before the route number, like N13, N212, and other similar routes.
Tickets may be purchased when boarding the bus directly from the bus driver. One may pay in cash, ideally paying in exact change, or else by using a contactless credit/debit card and tapping it on the reader next to the driver.
A cash ticket bought directly on the bus may be used for a 2 hour timeframe. Within this time period you may board other buses by validating the ticket on the ticket machine close to the driver.