How to model A Shop Database
Not specific to your question but in general do not put sales into an expenditure table. The name of the table suggests that the table contains information about money leaving the business, not coming in - it will confuse the reader and hinder maintainability in the future. I suggest you create a second table called sales and put your receipts in there.

There are very good example database designs that you can find online (I googled bookstore database schema and quite a few hits came up that would be useful to you).

Additionally, if you were going to do this seriously, then I'd look into the general ledger system or otherwise known as double entry book-keeping system which will help you model this a bit better. I'm not an accountant, but wikipedia seems to have a decent link on it:

https://en.wikipedia.org/wiki/Double-entry_bookkeeping_system