My kids also eat two or three at a time. Mandarins don't last very long, so I let them eat them as they like.
I think going to the tax office is a good idea. Etax is good in that you can do it at home, but it's still not the easiest to use. Last year I had trouble getting the apps to work together and had to repeatedly log in and log out which was very frustrating 😂
RE: Mandarin oranges as a tax return gift / ふるさと納税返礼品の蔵出しみかん