Pinpoint all the toll locations in Stockholm on the Waze map so that the driver easily can see if the calculated route will pass it. It will be more clear than just saying that there is a toll. Because then if you want, you can just select the route that passes the least amount of tolls, instead of gambling on the estimated value that you provide now.

You can find the exact locations of Stockholm tolls here: https://www.transportstyrelsen.se/sv/vagtrafik/Trangselskatt/Trangselskatt-i-stockholm/Betalstationerna/Betalstationernas-placering1/

Estimate the tolls cost more accurately. You can find the cost for passing the tolls at different times during the day here:
https://www.transportstyrelsen.se/sv/vagtrafik/Trangselskatt/Trangselskatt-i-stockholm/tider-och-belopp/