According to the Rules of Procedure Governing Supply and Service Contracts Below the EU Thresholds (Unterschwellenvergabeordnung, UVgO), direct contracts are given for

“services that can be procured up to an anticipated contract value of 1,000 euro (excluding VAT), taking into account the budgetary principles of cost-effectiveness and economy, without conducting an award procedure.”

Based on section 3 (3) no. 1 of the E-Invoicing
Ordinance (E-Rechnungsverordnung, ERechV), direct contracts are excluded from the general obligation to issue electronic invoices under section 3 (1) ERechV.