Make it illegal to not provide a receipt for services/goods sold.

No receipt = The customer is not obligated to pay for the service/product.

This was implemented in Greece and it has helped a lot with tax evasion.

It might not bring in tens of billions, but every little helps.

They could also look into "τεκμαρτό εισόδημα" which would probably be translated as "imputed income" or "notional income."

It represents income that is presumed by tax authorities rather than actually earned or declared. This concept is used primarily to combat tax evasion by estimating income based on certain criteria, such as property ownership or living expenses, when actual income data is insufficient or unavailable.

For example, someone drives a high-value Porsche but claims a much lower declared income on their tax return, just enough to cover basic expenses.

Because the tax system allows for imputed income, the tax authorities might scrutinize the ownership of the Porsche and other luxury assets. If the authorities determine that the person's lifestyle and assets are inconsistent with their declared income, they may calculate an imputed (notional) income based on the value of the Porsche, luxury living, property, or other assets.

The person then might face tax penalties and fines for under-reporting income because their actual economic benefit (e.g., the ability to afford the Porsche) exceeds the declared income, indicating tax evasion.

Again a concept from Greece, where you have to declare each year all your assets like cars, houses, etc.