Hello.
The main problem with the Tax Clearance of Invoices Act is the definition of "cash payment", which includes all electronic transactions using external services (PayPal, ...).   However, these should not be equated with general "cash" transactions, as they are a completely different form of business.
A detailed description of the problem is described in the following blog
http://www.alesspetic.si/2015/05/davcne-blagajne-in-konkurencnost.html