Why do I see the error "Payment Error: (10002) - Security error - Security header is not valid" when trying to make a payment?

PayPal error code #10002 means that PayPal cannot verify your "security header" information, otherwise known as your API username, password and signature. If you are seeing the error message "Payment Error: (10002) - Security error - Security header is not valid" at the top of a payment form when trying to submit a payment, it can mean one of two things:

1. Your credentials are not correct. Try re-entering your API username, password and signature and make sure any spaces or extra characters are removed. 
2. Your account live/test mode does not match the form live/test mode. Check in PayPal to make sure your account is in Live mode and make sure your Soapbox forms match (test forms with Sandbox account credentials, live forms with live account credentials). 

Hopefully this will fix things right up, and you can sit back and start watching the payments come in!