A contracted price for the duration of the programme, not a price renegotiated at every phase.
A phased delivery schedule, set against your progress. Delivering 200 doors at once to a site with no dry storage is manufacturing a problem.
A homogeneous lot. Doors for the same programme are made from the same batch of material — the only way to guarantee that the door in the last flat matches the one in the show flat.
A single contact, who knows your programme and doesn't make you re-explain the phasing on every call.