How can I make custom LEGO packaging?
Maybe a search for custom box design although I doubt they'd make small volumes either. It would look very professional, though.

What I've seen fans selling custom kits doing is using simple white boxes with stickers on them. Usually the buyers are more interested in the contents anyway. Sticker printing shouldn't be a problem (as it's going to be put on cardboard a simple inkjet-sticker sheet should do), and I assume plain white boxes shouldn't be too hard to find.

An laternative would be to print your images on cardboard which you can fold yourself, but then you'll need to think properly about placement of images. If the cardboard is thin, you can use it as an outer shell for an inner white box which would be more solid. 3mm cardboard may be overkill, but that's the general idea.