Hi Asad,
I'm not able to reproduce this problem. Web Invoice Pro is designed to allow the user to pay the invoice without having to sign in to the site. When I test as an unauthenticated user I'm able to search for an invoice and click the found invoice link and proceed to checkout. The only thing that would cause a redirect to access denied is if the user is not in the allowed view roles for the page, but then he would not be able to see the invoice search, so I don't understand how you are able to produce this problem.
Best,
Joe