[aspratley]

You don't need an invoice for a company expense. How do you think people claim back for things bought in a shop?

[hierro]

Not everyone is in US. See my comment at https://news.ycombinator.com/item?id=7086631. If you want to claim an expense for something you bought in a shop, you need to ask for an invoice at the time of the sale. In fact, even non-business entities can ask for an invoice (even if it's useless to them) and is illegal for the seller to refuse.

[riffraff]

In some(? european?) countries you can't deduct stuff bought in a shop from taxes unless you have an invoice.