The short answer is: YES we would very much like to integrate such functionality. In fact your request seems not too complicated ... It would require a "cart" feature somewhere, and a "checkout" button. When the user checks out, basically it would send an email to you with reference links to the images.
The slightly longer answer is: It's COMPLICATED, because different users seem to have entirely different requirements in this department. Some users want to be able to select print-styles, and also link to a payment-service. Without going into too much detail, the order/payment part is immediately a much more complicated aspect for numerous reasons.
Here is a related thread, basically discussing "shopping-cart" integration:
https://forum.photo.gallery/viewtopic.php?f=53&t=8710
But yes in general your suggestion sounds like a basic "cart" or "lightbox" feature, which may fulfill the requirements of some website-owners. We will start looking into features like this in 2 months time after we have launched the new www.photo.gallery website and docs.