如何将我通过JavaScript计算的自定义金额作为金额发送到PHP表单

这是我的JS代码，它计算总额：

function calTotal() {
  var a = 250.0,
    b = 175.0,
    c = 90.0,
    d = 415.0,
    e = 225.0,
    f = 225.0,
    g = 225.0,
    h = 225.0;
  var sum = 0.0;
  if (booking_form.faat_item1.checked == true) {
    booking_form.faat_item1.value = a;
    sum += a;
  } else booking_form.faat_item1.value = 0;
  if (booking_form.faat_item2.checked == true) {
    booking_form.faat_item2.value = b;
    sum += b;
  } else booking_form.faat_item2.value = 0;
  if (booking_form.faat_item3.checked == true) {
    booking_form.faat_item3.value = c;
    sum += c;
  } else booking_form.faat_item3.value = 0;
  if (booking_form.osfa_item1.checked == true) {
    booking_form.osfa_item1.value = d;
    sum += d;
  } else booking_form.osfa_item1.value = 0;
  if (booking_form.osfa_item2.checked == true) {
    booking_form.osfa_item2.value = e;
    sum += e;
  } else booking_form.osfa_item2.value = 0;
  if (booking_form.osfa_item3.checked == true) {
    booking_form.osfa_item3.value = f;
    sum += f;
  } else booking_form.osfa_item3.value = 0;
  if (booking_form.osfa_item4.checked == true) {
    booking_form.osfa_item4.value = g;
    sum += g;
  } else booking_form.osfa_item4.value = 0;
  if (booking_form.osfa_item5.checked == true) {
    booking_form.osfa_item5.value = h;
    sum += h;
  } else booking_form.osfa_item5.value = 0;
  return sum;
}
function sendTotal() {
  var total = calTotal();
  return total;
}

这个代码计算结果很好，但我希望这个计算结果作为金额直接发送到我的PayPal，下面是我的按钮代码：

<form
  name="_xclick"
  action="https://www.paypal.com/cgi-bin/webscr"
  method="post"
>
  <input type="hidden" name="cmd" value="_xclick" />
  <input type="hidden" name="business" value="xyz@xyz.com" />
  <input type="hidden" name="currency_code" value="GBP" />
  <input type="hidden" name="item_name" value="MT1" />
  <input type="hidden" name="amount" value="sendTotal()" />
  <input
    type="image"
    src="http://www.paypalobjects.com/en_US/i/btn/btn_buynow_LG.gif"
    border="0"
    name="submit"
    alt="Make payments with PayPal - it's fast, free and secure!"
  />
</form>

企业名称已经更改，我试图直接使用这个sendTotal()，但PayPal给出了格式错误，请任何人帮助如何将我的JS计算的总额作为金额发送给PayPal。