And that is the new, more relaxed situation. At first, digital invoices were required to be signed with a qualified signature, i.e. spend a few hundred quid a y

And that is the new, more relaxed situation. At first, digital invoices were required to be signed with a qualified signature, i.e. spend a few hundred quid a year on certificates by a few select CAs (only the usual german "suspects" could qualify due to intentionally onerous requirements).