Don’t assume you need to say anything. Email inboxes are being flooded with company COVID-19 policies that no one cares about (or reads).
If your business doesn’t have face-to-face contact with your customers, there’s not much to say. Of course, you can inquire about their health and well-being. But that’s a relationship issue, not a legal one.
To the extent your business does have in-person contact with customers, you should make it clear the steps you’re taking to limit the risk they’ll get the virus while dealing with you. This may include letting them know the cleaning steps taken for your premises and by your employees. If you’ve changed the way you do business (e.g. offering curbside pickup), tell them. And if you’re providing protection for in-store visits, you may want to offer items to limit exposure, i.e. hand sanitizer, sanitizing wipes, complimentary N95 air masks, disposable gloves, etc.