Go 商品的价格计算

[Documentation](https://godoc.org/github.com/shopspring/decimal)

[Github](https://github.com/shopspring/decimal)

price, err := decimal.NewFromString("136.02")
if err != nil {
   panic(err)
}

quantity := decimal.NewFromFloat(3)

fee, _ := decimal.NewFromString(".035")
taxRate, _ := decimal.NewFromString(".08875")

subtotal := price.Mul(quantity)

preTax := subtotal.Mul(fee.Add(decimal.NewFromFloat(1)))

total := preTax.Mul(taxRate.Add(decimal.NewFromFloat(1)))

fmt.Println("Subtotal:", subtotal)                      // Subtotal: 408.06
fmt.Println("Pre-tax:", preTax)                         // Pre-tax: 422.3421
fmt.Println("Taxes:", total.Sub(preTax))                // Taxes: 37.482861375
fmt.Println("Total:", total)                            // Total: 459.824961375
fmt.Println("Tax rate:", total.Sub(preTax).Div(preTax)) // Tax rate: 0.08875