I had a similar task when I was working in my mom's consignment shop. except it was clothes, jewelry, and consigners. not something one would normally expect a highschooler who couldn't even keep track of their art supplies, much less anything else to be able to do. excel is both a savior, and a nightmare, but I couldn't even image doing any of what your job entails on paper!! I practically lived in the local Ace Hardware when my grandma was building her house. it was a small building, and there never seemed to be a moment for anyone working there to rest.
Also! Your mother-in-law sounds like a very nice, and very wise woman. I had to lose the consignment notebook 4 times before I was allowed to touch the store's laptop. (it wasn't on purpose! I swear! they kept getting mixed in with my homework, because I can't work without doodling on something, so every notebook I touched came away doodled)
anyways, it sounds like it's tiring, but enjoyable work! Thankies for the update, and always remember to take your time and rest well ^_^