Compulsory invoicing for every work, service or sale of goods - no exceptions, simplified business for micro-taxpayers and strict on-the-spot inspection with high penalties for non-issuance of invoices.
The State should allow the unemployed, students, pensioners, gardeners, forest owners, etc., to provide for their own social security by carrying out small-scale work and selling their products to private individuals up to a certain annual income, with simplified business (if the work is not prohibited by a specific regulation). Once this limit is exceeded, the taxable person must, for example: set up a company; a farmer becomes liable for bookkeeping; an unemployed person loses the right to a cash allowance; a pensioner's pension is frozen.... An invoice must be issued - also for the sale of produce at the market, door-to-door sales, firewood, help between neighbours, instruction, domestic help, farm help, etc. Work or goods must be invoiced, even if the invoice is settled in goods or services.  The only exception is work within the family (spouse, parents, children, grandchildren).
Suggestion: The taxable person should declare the small work at the VEM entry point or at the DURS branch and take out insurance against work-related injuries (contributions can be paid for the whole year in advance, without the obligation to fill in the REK forms). Here, he/she should be given a voucher with a validity period, which he/she uses in the field to prove that he/she is working legally, and should pick up a paragon block of numbered invoices in duplicate. He should keep only a book of invoices up to a certain amount, make a statement of income at the end of the year, give the invoice numbers and pay the tax after deduction of the flat-rate charges.