To set up PayPal Express you must have a free PayPal Business account and an API Signature.
To create your PayPal API signature
1. Log into your Verified PayPal Business Account
2. Select the tab Profile
3. Click on API Access
4. If you have not done so already click on Request API Credentials
5. In the next window choose the API Signature Option and Submit
Once you have the API key log into your Santu account and go to My Store / Payment methods (http://app.santu.com/buynow/store/payment)
Click on Edit methods, select PayPal Express and then enter your details - i.e. the API username, API password and the signature.
Then complete the Wizard.
PayPal Express should now show up on your Shopping Cart page as additional checkout button.