Most likley the collection agency will try to collect their additional charges, however
in addition to the basic claim, only default interest is owed. Reminder fees are owed if this is provided for in the general terms and conditions of your service provider. Any additional charges do not have to be paid hence can be contested.

So basically you still owe interest on the original amount
and possible reminder fees.

https://www.srf.ch/sendungen/kassens...nd-ihre-rechte