This is the kind of thing that's made for the tickler file.
I've always had a reminder to file my expense report. I have it recurring once per month on OmniFocus, but you could also do a calendar reminder, index cards, whatever.
Back when I had a paper system, it used to help for me to drop my receipts into the folder on the day I processed my expense report. That way, when the day came, all my receipts were right there in the folder.
If they frequently fill up their tanks, I'd recommend a once-a-week reminder to process their receipts so the task doesn't seem unmanageable (oh, now I have to go gather up my receipts).
Of course, whether they actually do that is up to them, but that's more of a management problem - I think the best thing to do may be to set a deadline and explain why that deadline is there.
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