DDP quote to Germany keeps blowing up on customs — who's eating this cost?

Running a 12-cavity hot runner mold project for a Baden-Württemberg Tier 1, quoted DDP Stuttgart because the buyer pushed hard for a landed cost. First shipment cleared fine. Second container — same mold base, plus spare inserts — got flagged at Frankfurt customs and they hit us with a reclassification on the steel inserts, claiming they're separate dutiable goods, not part of the mold. Extra EUR 4,300 in duty and storage, sitting there 9 days now, and the customer is politely asking who pays. I've tried splitting the invoice, added the HS code on the packing list, sent the BOM proving the inserts are mold components. Still stuck. Anyone shipped DDP Germany with spare parts bundled and got through clean? Do you go DDU and let the buyer be IOR, or is there a broker trick I'm missing? Losing margin fast on this one.