关于c＃：计算出纳计划中的销售税

Calculating sales tax in cashier program

为什么增值税不计入小计和合计？

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//Value from xQuantity
double quantity = Convert.ToDouble(xQuantity.Text);

//Value from xUnitPrice
double unitPrice = Convert.ToDouble(xUnitPrice.Text);

//Value from xSubTotal and xTotalPrice
double SubTotal = quantity * unitPrice;
xSubTotalTextBox.Text = Convert.ToString(subTotal);

double Tax = subTotal * 1.06 + subTotal;
xSubTotalTextBox.Text = Convert.ToString(subTotal);
xTotalPrice.Text = Convert.ToString(totalPrice);

//Calculate subtotal and totalprice
subTotal = Convert.ToDouble(xQuantity.Text) * Convert.ToDouble(xUnitPrice.Text);
xSubTotalTextBox.Text = subTotal.ToString();
xTotalPrice.Text = (subTotal * Tax).ToString();
}

private void xBalance_Click(object sender, EventArgs e)
{
xRetTextBox.Visible = true;
xReturn.Visible = true;

double totalPrice = Convert.ToDouble(xTotalPrice.Text);
double receive = Convert.ToDouble(xRecvTextBox.Text);
double subTotal = totalPrice - receive;
xRetTextBox.Text = Convert.ToString(totalPrice);

if (totalPrice < .01) xRetTextBox.BackColor = Color.Green;
else xRetTextBox.BackColor = Color.Red;
}