Tell them no. If 0.0005% isn't good enough, and they don't care enough about your business not to force you to incur substantial costs on 1 in 200,000 error, they can find someone to do it better than 0.0005%.
Have they ever been late (even by a second) in payment? Demand an irrevocable demand letter of credit to back their payment. Oh, it's going to cost them bank fees? Too bad. If any slip up is material, then we'll need full bank support. Oh, and personal guarantees from the owners.
If it's not set forth in whatever contract (including a purchase order) you have, tell 'em to screw.
Alternately, tell them they've got to have one of their people present for the full duration of the segregation, and if less than 0.001 percent are faulty, they'll pay the full cost of your time.
They'll come back.
JP