eCART and PayPal processing

Is there anything that I should be looking at when I process paypal transaction it comes back with "Transaction Failed". How do you debug that error?

See the "Debugging eCart Transaction Failed issues" tutorial in t he How To section of the eCart support page:
ecart/

This has steps on adding bindings to the failure page to get information back from paypal to find out why the transaction failed.

So I added the new bindings per your suggestion and got the following (API key edited for security reasonds)for the two I added per the "Debugging eCart Transaction Failed Issues" you recommended I implement...

Transaction Not Completed
We're sorry
Your transaction could not be completed.

Could someone please call me so I can give the API info and walkthrough the issue? 908-601-1609

The security header is not valid error message means that you are not using the correct API Credentials.

if you request live Credentials and set the transaction to use the sandbox server, you will get this error, or if you are using sandbox API credentials and connecting to the live server you will get this error.

Also make sure that you have accepted the billing agreement with paypal after requesting the API credentials.

I spoke to PayPal and my account is setup correctly. The error theyt said would be produced when using sandbox credentials with a live account or vice-versa. In looking at the code in the checkout page I found the following following my live api credentials...

$nextIndex = count($WA_PP_ECO_Set_params[0]);
$WA_PP_ECO_Set_params[0][$nextIndex] = "UseSandbox";
$WA_PP_ECO_Set_params[1][$nextIndex] = "false";

is there a way to change the above param to not point to the sandbox? Or what is it really doing there? Please advise asap so I can get this site live...we're losing sales without the site up.