It would be really nice if you could see what items you have donated to the museum through a tab in your inventory (similar to the bundles tab) that you could click to see your donated items (with item names of course).
maybe at the tabs of the items you shipped/found? i found out that it doesnt show you everything in the museum, but what you found i thought to NOT give the diamond to Gunther yet, and still saw it in my tab of items maybe a simple "Donated to the museum" would do