@DhavalPX has the technical part right. But I think the more interesting part of your question is the business risk, not the platform choice, and that's worth addressing separately.
The concern about "giving up part of your business" is valid, but I'd flip it around. The bigger risk is telling a client "we can't do that." Because once they go find a POD vendor who can handle one-off branded items, that vendor now has a relationship with your client. From there it's a short step to "Actually, can you do our bulk orders too?" You're more exposed by not solving it than by solving it with a third party.
The architecture most shops use for this hybrid situation:
- Keep Pressero (or whatever bulk platform) for inventory-backed orders, minimum quantities, and your core fulfillment
- Use a white-label or behind-the-scenes POD partner for the one-off promo items, something like Printful or Printify, for commodity items (t-shirts, bags, standard promo gear). The client never needs to know who's producing it.
- Keep the ordering experience on your side. Even if you're just acting as the integrator, the client portal, the approval flow, and the invoicing all come from you.
The key is making sure the client never has direct visibility into who the production partner is. As long as your shop is the point of contact, you stay in control of that relationship.
One thing worth thinking about: Is this a one-client ask, or are you seeing this pattern more broadly? If multiple clients are asking for one-off branded items, that's a signal your fulfillment model may need a structural adjustment rather than a one-off workaround. A lot of print and promo shops are quietly running a hybrid setup now, even if they don't advertise it that way.
Shopify as a platform is fine for the public-facing storefront side if you want to spin up something for that client specifically, but it's more overhead than the problem warrants if you're just handling occasional one-off orders. A POD integration sitting behind your existing client portal is usually the cleaner route.