The whole concept of having to 'order' and wait for something digital online seems to be missing the point...
Imagine if Google decided that to do a search I first had to add it to my basket, check out, and then wait for my search to be 'delivered' via a link in my email...
You are making the assumption that the thing you are ordering is already available in digital format. This is a National Archive, most of their historic holdings will be in paper form. There is a not insignificant cost to digitisation, so they may be digitising on demand for you.
Imagine if Google decided that to do a search I first had to add it to my basket, check out, and then wait for my search to be 'delivered' via a link in my email...