How to set a white point for customers to both view online and then order prints directly from the website?
If you use any other white point, you risk metameric failure when prints are viewed under standard D50 conditions.
On the other hand, D65 has become a more or less de facto standard for many in the graphics industry where most everything is viewed on a monitor and no one is worried about printing anything. Many mobile devices aim for D65.
In either case, you have no way of guaranteeing that your clients will be viewing online photos on a D50 or D65 compliant device. You also have no way of knowing the lighting conditions under which they will view and display purchased prints.
The ISO standard for viewing prints and for viewing images on a monitor (D50) is what should guide your choice. The best you can hope for is that the viewer of your images have also created a viewing environment that complies with the same standard.
¹ The standard is ISO norm 3664:2009. You'll need a registered (paid) user account to view it directly online at the ISO website.