Entrepreneurs have been obligated since 1 January 2014 to have an entry certificate for every goods export within the EU (so-called intra-Community supply) and to archive this. Recently, the tax offices are placing more and more importance on this and having entry certificates handed out during company audits.
Only if an entry certificate exists with the following components is VAT-exempt billing in the other European country justified and recognised by the tax offices. However, there is not a standard formal regulation for this. You now have the choice of issuing your own entry certificate for every sale or alternatively a collective certificate for sales up to a quarter.