Hi there,
I did the similar project as you described. Please check my recent project and also the live website below.
Did many PayPal and other payment method projects recently, please check my reviews:
https://www.freelancer.com/u/coder786sl.html
Live Website: [login to view URL]
Note: I did three processors there, PayPal, Stripe and Bitcoin.
I have also downloaded your document file and read it carefully for the step by step guide you provided for checkout process.
Looking forward to hear you soon!
Best Regards,
Kashif (coder786sl).