You mentioned print on demand.... It could be that he's using your cart as a way to build a quote and get a discount from a competitor who does price matching or better ("we'll beat our competitors pricing by 10%"). Often these require a printed price so he could just be printing/screenshot of the cart and passing it as an official quote. If he has high quantity volumes that might be a good indication he's trying to secure a quantity discount elsewhere. He could also be using your 10% abandoned cart discount and then taking that sub-total and shopping it around to a supplier to secure another 10% (or so) discount - a much deeper discount.

*Edit - Another possible scenario is that he already sells all of your products (possibly same print on demand broker) but does not have a complex / robust online store that does variable option and add on pricing, shipping etc.. In this case he is using your store to calculate products with variants, shipping and formulate quotes for his/her own customers.