I might go with "Office Supplies" and list the umbrella as the detail. If the company doesn't think that's appropriate they can question or deny that line. I wouldn't lie about it.

Office Supplies is kind of a broad category when out of the physical office. Think of it as a shorter version of the phrase "Accessories I need to do my job properly that aren't covered by other categories."

I wouldn't expense it for a trip to the airport, but if I was going to be exposed to the rain on my way to meet a client, then I might.

Regardless, I wouldn't go to the wall to fight for this expense from the company.

Cromely.