Hi Judy, Great question! We ask that slides be grouped into sets of 25 (one item) either in an envelope or ziplock bag. We will provide small barcode stickers to apply to each bag of 25. Slides will be processed in numerical order, according to those barcodes. The order within each set of 25 is not guaranteed as there are alternate factors such as shipping where they could move around. The digital files are free from any copyrights so you can very easily re-order them however you'd like once you receive them back! Please reach out to our customer service team for the fastest responses at questions@legacybox.com if there is anything else we can help you with!