My shopping cart system requires that my API is linked via Certificate with PayPal Payments Pro rather than a signature. Because of this, and since PayPal only allows 1 option for API access I am unable to integrate with ZOHO as it requires the signature option. I am sure that other people are in the same position as me, I think offering the option of either or is a a great thing to integrate so that users in my position can better utilize the amazing zoho books system.