Show the agreed spare-kit contents against the receiving shortfall, because the additional adapter is not satisfied by the one fitted to the arm, and ask the parts contact to address the whole list rather than whichever item sales happens to mention.
Keep the receiving check attached. Main-package receipt and complete spare-kit receipt are separate entries; the same delivery reference need not close both.
When that owner replies, ask whether the contents are available and what dispatch terms apply. Or rather, ask those as separate questions; accepting a shortage still won't tell you whether they can supply it.
A commercial alternative might be worth hearing too, provided it covers the actual kit. Don't accept a token credit because you're tired of the emails, then discover the replacement adapter alone costs more than the whole offer.
Carla, yes, though an alternative price is information for Elliot's decision, not an instruction to give up the owed supply before anyone has checked what can actually be sourced.
Have they stated the credit amount and whether accepting it settles every missing line? You need definite terms before comparing it with a confirmed replacement quotation.
Then leave the choice open until that quote is real. Check availability and delivery cost too, not just the adapter price. A cheaper line item with no supply date could leave maintenance in exactly the same position.