Passing a cart total variable to Paypal in asp.net - asp.net

Question

When implementing Paypal I have managed to set it up to send a set value such as £25 however not a variable from the basket e.g. 'GrandTotal' which is dependent on what the customer has entered into the cart (I have created my own cart.)
<form action="paypal.com/cgi-bin/webscr"; method="post">
<input type="hidden" name="cmd" value="_xclick">
<input type="hidden" name="business" value="you#youremail.com">
<input type="hidden" name="item_name" value="Item Name">
<input type="hidden" name="currency_code" value="GBP">
<input type="hidden" name="amount" value="25.00">
<input type="image" src="paypal.com/en_US/i/btn/x-click-but01.gif"; name="submit" alt="Make payments with PayPal - it's fast, free and secure!"> </form>
Do you know how to send the variable rather than a set amount? Is the code below anywhere near a solution?
<input type="hidden" name="amount" value="GrandTotal">

Answer 1

You cannot insert your form tag in ASP.Net page (unless iframe), because ASP.Net only allows one form tag in a page.
You want similar like this in code behind. It basically collect user entered Amount and Quantity, and then post it back to PayPal.
protected void PayButton_Click(object sender, EventArgs e) {
string paypalUrl = IsTestMode ?
"https://www.sandbox.paypal.com/us/cgi-bin/webscr" :
"https://www.paypal.com/us/cgi-bin/webscr";
var builder = new StringBuilder();
builder.Append(paypalUrl);
builder.AppendFormat("?cmd=_xclick&business={0}",
HttpUtility.UrlEncode(you#youremail.com));
builder.Append("&lc=US&no_note=0&currency_code=GBP");
builder.AppendFormat("&item_name={0}",
HttpUtility.UrlEncode(YourItemName));
builder.AppendFormat("&amount={0}", AmountTextBox.Text);
builder.AppendFormat("&return={0}",
HttpUtility.UrlEncode("http://mysite.cm/ReturnUrl"));
builder.AppendFormat("&cancel_return={0}",
HttpUtility.UrlEncode("http://mysite.cm/CancelUrl"));
builder.AppendFormat("&undefined_quantity={0}", QuantityTextBox.Text);
HttpContext.Current.Response.Redirect(builder.ToString());
}

Answer 2

You can make input field server side by adding runat="server" in input field tag and can set it value from .cs file
field.value=GrandTotal;
Your aspx page code
<form name="_xclick" target="paypal" action="https://www.paypal.com/cgi-bin/webscr"
method="post" runat="server">
<input type="hidden" name="cmd" value="_xclick" />
<input type="hidden" name="business" value="you#youremail.com" />
<input type="hidden" name="item_name" value="Item Name" />
<input type="hidden" name="currency_code" value="GBP" />
<input type="hidden" name="amount" id="amount" runat="server" />
<input type="image" src="https://www.paypal.com/en_US/i/btn/view_cart.gif" border="0"
name="submit" alt="Make payments with PayPal - it's fast, free and secure!">
<input type="hidden" name="display" value="1">
</form>
Your code behind page code
protected void Page_Load(object sender, EventArgs e)
{
if(!IsPostBack)
{
amount.Value = "25.25";
}
}