Passing data to CGI script after form submission

I’m unable to find a solution to this question, although I am aware it may be because it is a pretty simple one.. Nevertheless, I’d really appreciate some help.
I have a custom-built payment processing plugin (for CCBill) on a client’s WordPress site. After an initial membership payment, users can choose to make additional donations in any amount, which are directed through CCBill’s ChargeByPreviousTransactionID system, so they don’t have to enter their card details again.
The plugin checks to see if the user has an existing transaction and pulls the relevant data from the database. Then when they enter an amount and submit the form it constructs a query string, and then loads this as the url of a pop up window.

This causes two specific problems for us:

• most browsers block the pop up, &
• the IP address submitting the query must be whitelisted at the payment processor for the payment to go through – this should be the IP of the site, but with the pop-up window it appears to be the user’s IP, which we can’t whitelist

Is there a way to pass this data to the script, without opening a pop up, or having users leave our site?
I can’t get the original developer to fix this issue, and I would really like to improve my programming skills by being able to fix it myself.

Here are the relevant parts of the code:

function shortcode_function(  ) 
{
    $detailsid='';
    if(is_user_logged_in())
    {
        global $current_user;

        $detailsid = get_user_meta( $current_user->ID, 'get_donate_details_id', true );
        $detailsmoney = get_user_meta( $current_user->ID, 'get_donate_details_price', true );
        if($detailsid!=''&& isset($_POST['wq_re_donate_now']))
        {

        ?>
            <script type="text/javascript">
            var url="https://bill.ccbill.com/jpost/billingApi.cgi?clientAccnum=900000&username=test123&password=test123&action=chargeByPreviousTransactionId&newClientAccnum=900000&newClientSubacc=0000&sharedAuthentication=1&initialPrice=<?php echo number_format($_POST['wq_amount']); ?>&initialPeriod=365&recurringPrice=0&recurringPeriod=0&rebills=0&subscriptionId=<?php echo $detailsid; ?>&currencyCode=840";

            var myWindow =window.open(url, "myWindow", "width=200, height=100");
            </script>
            <?php
        }
    }

if($detailsid!='')
    {
            ?>
    <div class="donateholder">
    <div class="donateheader"><h4>Make Donation</h4></div>
    <form method="post" action="">
    <div class="donatediv" align="left">
    <h3>Amount: $<input maxlength="4" onkeypress="return isNumberKey(event)" type="text" size="5" class="amount" name="wq_amount" id="randomnumber" />
    <input type="submit" name="wq_re_donate_now" value="Submit"  />
    </h3></div>
    </form>
    </div>
<?php
    }
}

Thanks!