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All the content above is take from the the PayPal Overview page here, https://developer.paypal.com/docs/checkout/.
Setup:
In order to configure PayPal Smart Payment Buttons we need you to create a “Straightsell” App in your PayPal account and provide us the Client ID and Secret for this new App.
To do this follow these steps:
Go to https://
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developer/applications and select “Login to Dashboard” at top right to Login,
On the “My apps & credentials” page, toggle to “Live”, then select “Create app”.
Add an App Name called “Straightsell”, and select the “Create App” button.
Select “Show” Secret, then select and copy the “Client ID” and “Secret” into an email and send it in to your Straightsell Project Manager.
Log out of PayPal by choosing your name drop-down at top right.