When you work with customers in retail, whether face-to-face, online, or over the telephone, there is no underestimating the importance of good customer service. Even if customers need your products and are regulars at your shop, if you provide them with poor customer service, they will quickly take their custom elsewhere.
Not only would this be terrible for your reputation, but it will also quickly impact your revenue and your profit. The last thing you want is to provide your customers with is a bad experience, and if they stop shopping with you, you may quickly find yourself losing other customers as well.