our extra ur5e cable is still always on the next shipment

Balance and cable reference agreed already, still no stock answer

Then I'd stop resending proof of what's missing. Ask what they can actually offer. I managed to buy a beautifully narrow test recently by not reading the scope closely enough; I'd read any credit offer for the whole outstanding balance, not just the headline amount.

They've offered credit for the complete missing spare kit, with no date for replacement supply

Do you have a checked alternative source for the spare cable, or would accepting the credit leave you restarting the availability search?

And check any replacement cost with the person responsible for maintenance cover. A credit can settle the order while leaving the workshop's spare provision unchanged

Maintenance found the correct kit available locally and checked the contents; it's dearer, but our budget owner prefers that to an undated supply

That sounds like a choice with the awkward bit visible. I'd put the difference on the order decision so nobody later mistakes it for an unexpected overspend.

Credit received and local kit delivered, contents checked into stores; agreed price difference recorded too, so this one's finished