I know its too late to suggest now, but of course who knew what you were going to do, but it seems that working within the context you describe you could make two orders.
1. photos at a 3:4 ratio (standard digital camera ratio)
2. photos at a traditional 2:3 ratio.
Two separate orders might not confound the software
I shouldn't really be necessary to have to make two orders. You request one size print for all of the photos you upload, the photos should be cropped to this size if necessary.
The frustrating thing is that Aldi's own uploading software indicated that the 3:4 ratio photos would be cropped. But this didn't happen!
The safe bet is to obviously just upload one photo size and ask for one print size. Minimise the chances of them f#cking up.
It's an issue that Snapfish appear to be well aware of however and it doesn't seem to be a problem for them: [broken link removed]
TIP: If your order contains both digital format 3:4 and classic film format 2:3 images and you choose digital format print option for your digital format photos, your classic format prints will be slightly longer than your digital format prints. Choose classic format to receive all same size prints.