Hidden cost 1 — Sample orders (mandatory for quality control)
POD platforms strongly encourage 'order a sample before going live' for every new design. They're right — color, placement, and fit drift between mockup and physical product is real and unpredictable. What they don't put on the cost sheet: a sample is a full retail purchase at your discounted seller rate plus shipping, no margin on it ever.
Cost per design: $12–$18 (sample + shipping) for a standard shirt. Multiply by your design count. A shop with 40 active designs has spent $480–$720 on samples — none of that is recoverable. If you reformat a design or the supplier changes color batches, you re-order samples (another $12–$18 each).
Per-shirt amortization: across the lifetime of a design that sells 30 shirts, the sample adds $0.40–$0.60 to cost-per-sale. Across the lifetime of a design that sells 3 shirts (the median in most POD shops), it's $4.00–$6.00.