The Retrax XR tonneau covers have grooves on the sides that support rail mounting systems from e.g. Thule and Yakima. Here's a link to the one we have.
The first picture shows exactly how it works.
Note that the cover doesn't include any of the rails/crossbars. You have to buy those yourself.
Woof, I just saw the price. Either we got ours on sale or they raised the price a lot. There's a cheaper version that's plastic instead of aluminum. They may also be trying to steer you to a local dealer/installer, so if you like the product but not the price, you might try calling around.