Because print-on-demand products are made specifically per order rather than pulled from existing stock, standard retail return expectations (simply send it back for any reason, get a refund) don't map cleanly onto the POD model, and it's worth writing a return policy that's honest about this rather than either overpromising or leaving customers confused about what to actually expect.
Most POD platforms distinguish between a genuine defect or error (wrong item, damaged in production, significant quality issue) versus a customer simply changing their mind or ordering the wrong size — the first category is generally covered by the platform's own defect/replacement policy at little or no cost to you as the seller, while the second is often genuinely not eligible for a standard return, since the item was custom-produced specifically for that order and generally can't be resold to anyone else.
Being upfront about this distinction in your own policy, rather than vaguely implying standard retail-style returns are available, sets accurate expectations before purchase rather than creating a frustrating surprise after the fact. Something like "because each item is made to order, we're unable to accept returns for size or preference changes, but if anything arrives damaged or defective, we'll make it right" is both honest and reassuring at the same time — it's not simply a denial, it's a clear boundary paired with a genuine commitment where it actually applies.
For sizing specifically, this is exactly why the sizing FAQ and clear pre-purchase sizing guidance covered elsewhere matters more for a POD shop than it might for a traditional retailer with easier returns — since a customer genuinely can't just return a wrong size as easily as they could with a traditional retailer, getting the sizing information clear before purchase carries more weight in preventing the problem in the first place, rather than easily resolving it after.
It's also worth clearly stating your process for defect claims specifically — how a customer should report an issue, what information or photos you need, and a realistic timeframe for resolution — since a customer experiencing a genuine defect is already somewhat frustrated, and a clear, easy-to-follow process for making it right goes a long way toward preserving the relationship despite the initial problem.