If your vehicle is heavier than 3.5 tons, you must already have an electronic device - on-board unit - for transiting toll sections.
The toll for each toll box is deducted when the vehicle passes through the toll station. The rate depends on the class of the road, the length of the toll section, the day of the week, the number of axles on the vehicle and the emission class of the vehicle.
Where is the stamp needed: Roads where it is mandatory have a road sign (motorway, expressway). Except when there is an additional "free of charge" sign. Finding your way on toll-free sections is very complex especially for foreigners.
Where to get an on-board unit: Get and prepay for the unit (cash or card) at a distribution point. Note that they will want a security deposit.
More detailed information about tolls in the Czech Republic can be found in several languages at: www.mytocz.eu