Link with PayPal account

Hi there,

Thanks for the simple, clean and customizable plugin.

However, I can’t seem to find the way to link my PayPal account to this plugin, not even in the documentation. The only thing that I’ve keyed-in which is related to my PayPal account was the PayPal email address. I couldn’t find any other fields on the Plugin Option page that allow me to key in the API credentials.
As a result, I got the message “We’re sorry, things don’t appear to be working at the moment. Please try again later.” after clicking on the button.

Can you kindly enlighten me and perhaps some others who face similar issue as to what I may have missed out? Or is it necessary to have a shopping cart or to set up an e-commerce store in order to use this plugin?

p.s. I just needed a simple button to link customers to PayPal site and pay the amount that was invoiced to them.

Thank you so much!

The page I need help with: [log in to see the link]