We've finally reached the stage where we need to take Sandbox mode off, and have entered the credentials for the main API. Unfortunately, when I reach the payment section of the cart, I see This transaction is invalid. Please return to the recipient's website and try again.. This is the first time we've used PayPal's API/APP settings, so I may need some help 🙂
I've filled in the details as follows:
1, 2 & 3 – I got these logging into my merchant account, going to Profile > Selling Options > API access:
1 – API Username
2 – API Password
3 – Signature
For number 4, I went to developer.paypal.com & got the Client ID from Live Credentials.