I'd recommend a Dragonfly over the Delica for a few reasons.
1. It's smaller. Not that the Delica is big, but the smaller knife will always be less threatening than one that's larger.
2. Despite being smaller, it'll feel bigger in the hand. The finger choil, which the Delica lacks, gives you a more comfortable and controllable grip. I have a Kopa that's even smaller than the Dragonfly and it's more comfortable in my hand than the Delica.
3. Less obvious clip. The wire clip is harder to notice and looks less like a knife clip than typical ones.
Whatever knife you end up getting, get one that's brightly colored. Yellow or orange is probably the best. People associate these colors with tools rather than weapons. Most box cutters I see used by employees are brightly colored. If you go with a yellow Spyderco it'll be part of their Salt line which have blades that are impervious to rust.
I would also recommend getting an Opinel for use on boxes when you don't need a one-handed knife. The most common Opinel is the no.8, but IMO it's just a bit large for "professional" use. I'd get a 6 or 7. I prefer the 6 myself for its small size (smallest Opinel with the locking ring), but the 7 is more comfortable for me to use because I can get a full grip on it. The blades and handles on Opinels are proportional to their size. Meaning that they don't just make the handles and blades the same across the sizes. Handle diameter increases by number, as does the thickness of the blade. That's why I like the 6 so much for cardboard - it just goes through it like butter. Regardless of size Opinels are great with cardboard because they're all "scandi" ground; instead of a flat grind it curves from spine to edge giving it great blade geometry for slicing. And since they're ground so thinly to the edge the blade will still eviscerate cardboard (or whatever) even when the actual edge is dulled.
I prefer the Carbone (non stainless) as does the majority, but they're available with INOX (stainless) blades too if you're worried about rust. The low price (under $15) also makes them practically disposable. I prefer an Opinel over the disposable blade utility knives. The ubiquitous Stanly style utility knife is unnecessarily large and heavy and they rattle obnoxiously. The snap-off blade knives are too flimsily made. Opinels are solid.