All Products are on testing mode. Please use your paypal sandbox account to make the payment. If you don't have, you can use following PayPal sandbox account to test:
Username: james.liu128@outlook.com
Password: 123456789
HD Wallpaper at $0.99
Printable Coupon at $0.99
Linkin Park - New Divide
PayPal User Manual in PDF
Get PayPal Digital Goods Joomla Extension