Sites for On demand promo (shoptify??)

[JRow]

I have a client for whom we provide fulfillment services (print and promo) through an online ordering platform (Pressero). They’ve asked us to create a site where they can order one-off items (e.g., a single t-shirt, a bag, etc.). However, our setup is designed for bulk orders(inventory), not individual pieces (qty 1). Does anyone know of a third-party service that could handle this? Or is this the kind of request most of you would avoid? I’m always cautious about giving up part of our business, as it could create an opportunity for someone else to step in. Thoughts?