When taking a taxi from Palma de Mallorca Airport, the approximate fares to drive to popular destinations are displayed on the board, as shown below.
Although the end charge is much higher. For example the approximate tariff at night to Palma city or El Arenal is €19-20. However when you get to the destination the taxi meter shows €16 (where normally you expect to pay that amount), but then a driver clicks some button and 'suplementos' fee of around €5-6 is added and the taxi meter immediately shows €22-25 in total. Is it a standard practice?
What is 'suplementos' taxi fee exactly (showed below in red, but not much visible)? In other words how does it work (is it a fixed fee, or taxi driver can charge how much he wants)? If so, how to avoid overcharge? Especially in the scenario where there is no any traffic, no waiting time and around 10-15 min simple drive for 9-10km.