At a minimum, we recommend offering business hours customer service 7 days per week.  Best-case scenario is some form of 24/7 service, especially if your customers are spread out through different area codes (or travel to them). 

Keep in mind that you may want to budget for circumstances where the customer is not entitled to a refund/partial refund, but it might be in the business interest of your company to offer one. This is purely a business decision on your part, but experience with other clients shows that this can pay off in enhanced loyalty and positive social media exposure (and minimize complaints/bad press).