This concerns Widerrufsrecht, i.e. the right to cancel an order or contract shortly after signing it. For certain services you also waive this when you get to use it immediately (cellphone service, for example), and for custom-made items it also does not apply, as in this case.

This is independent of warranty (which is something the manufacturer may or may not offer), or Gewährleistung (which concerns the vendor and is typically the easiest way of dealing with damaged or defective goods).

Scummy approach, we have buyer protections for a reason.