It's 10% of 0 to 50 + 8.5% of 50 to 150 + 6% of 150 to 300 and so on. If the product costs $49, then it's 0.10 * $49 = $4.90 If it costs $52, then it's (0.10 * $50) + (0.085 * $2) = $5.17