The written policy or addendum, itemized receipts for every claimed expense, proof of payment, and any approval emails for costs that required sign-off. Keep them as the expenses occur rather than reconstructing the set weeks later when a submission deadline is near.
Build the claim file as you go
Reimbursement claims tend to fail on missing detail rather than on ineligible spending: a receipt without a payment method, a currency conversion nobody documented, or a category that needed prior approval. Keep a single folder with the policy at the front and receipts in date order. If a cost sits near the edge of a category, ask before spending, in writing.
Claims collapse most often on a single missing link between a receipt and the person who paid it: a card statement in one spouse's name, an invoice issued to the other, and no document connecting the two. Currency is the second recurring gap, where a conversion rate is applied months later and cannot be tied to the day of the transaction. Both come from filing receipts without filing the proof of payment beside them.
Photograph each receipt on the day, attach the statement line, and note in the folder which policy category the cost was claimed under and who approved it. Where a cost sits in two categories, record which one was used, since a resubmission under the other may be time-barred.