What Is The Best Way To Convert A Print Order Into A Digital One?

Scenario:  Person A has ordered 12 prints of 12 different images, from a gallery of 40 choices.  They have paid and all is well.

After thinking about it for a day, they decide they would also like the matching digitals for the images purchased, at extra cost to them.  I can organise the extra payment, no worries.

But without a lot of mucking about from me in the backend, or forcing them to make a new order, is there a simple way of re-sending that original order as downloads? 

You would simply have to email the photos to them.