Integrating Square.com Quick Pay into Website

To whom it may concern,

I sell traffic to small online businesses. I sell 2 types of traffic Login traffic and web traffic.
The attached screenshot is the form use to purchase Login credits. When a member enters the
number of login credits he would like to buy and clicks the Preview button, he should be
taken directly to the Square.com quick pay checkout page.

The codes are already set up to insert the product description, price, the member's username,
first name and last name.

On the checkout page, the member will enter his credit card details and click the submit button.
The data above should be sent to my IPN file. In this file I have the following code that
checks the transaction

if( ($_POST['payment_id'] !=null) && ($_POST['status'] =='COMPLETED') ){
include'config.php';
require_once("db_connect.php");

If both conditions are true, the member is redirected to the thank you page
and the two login tables are updated. The tables are
already in the database and the codes that update these tables
are already in the IPN file.

If both conditions are not true, a pop message should come stating
'This Transaction is Invalid. Please try again!" And the IPN file should
send the Invalid Transaction email to me. That
email is already in the IPN file as well.

Again I just need the Square.com Quick pay payment link linked to the
Preview button as shown in the screenshot.

Thanks