Whether a shirt at your price is genuinely worth printing — before you build a whole store around it.

Card fees of 2.9% + $0.30 are applied to the full amount the customer pays, including shipping.
Print-on-demand removes inventory risk, which is genuinely valuable. What it doesn't do is produce the margins most courses imply. A blank shirt plus printing typically lands around $10–14 before shipping, so a $22 tee is barely worth packing.
Under roughly 30% margin, a single reprint or refund wipes out several sales. If your numbers land there, raise the price before you build the store around it.