Bulk custom apparel becomes much cheaper per item mainly because production costs are spread across more pieces. Suppliers can buy fabric and printing materials in larger quantities, set up machines once for a large order, and reduce packaging and handling costs per garment.
For example, producing 10 custom T-shirts may require almost the same setup work as producing 100, so the setup cost per shirt falls significantly as the order quantity increases. Larger orders also give suppliers better purchasing power, which can further reduce material and labor costs.