This is a request we receive quite often and one that is very difficult to accommodate unfortunately. For technical reasons, we cannot add prints from different PDF files into a single shipment. It could be that the books get printed at different suppliers.
Each file has to be processed and inserted into the machines separately and that requires us to treat it as a separate order, all the way till packing and shipping. Nevertheless, we have made progress with some of our suppliers and expect this feature to be available across the whole of our platform soon.