UNIT ECONOMICS
Tax-exclusive revenue = displayed price / (1 + included-tax rate). Expected revenue = net revenue × (1 - return probability). Costs include all landed product cost, outbound shipping, gross-price transaction fee, flat fee and expected return handling. Contribution subtracts the acquisition cost. Break-even acquisition is expected net revenue less pre-ad costs.

Conservative assumption: product cost, outbound shipping and original transaction fees are not recovered after a return. Change the input model only with evidence from your own operations.