Have you ever placed an order with someone and then had to chase up at a later date as to whether they ever received it? Or, have you as the supplier been on the receiving end of the same email?
If a person doesn’t acknowledge that you want to spend money with them, it makes you, as the paying customer, feel undervalued. It is also more likely to happen with a long term supplier, rather than a new one. With a new client, people tend to be more attentive and likely to reply straight away, acknowledging their order, and often thanking them for it.