Margin math that actually predicts take-home profit
Most sellers compare only base costs, but true 2026 margin depends on the full cost stack: production cost, channel transaction fees, payment processing, and expected promotions. A one-dollar base-cost gap can still disappear if one platform requires heavier discounting to win conversions. The right decision is whichever platform leaves stronger net dollars per order after your normal selling conditions.
Keep your test clean by using one T-shirt style, one target retail price, and one marketplace at a time. That gives you an apples-to-apples margin read and prevents false winners driven by mixed assumptions.