Checkout by Amazon Zen Cart Plug-in

[AmazonPayments]

I installed the module today...however I'm receiving the following error when and order is placed. I make it all the way to Amazon but as soon as I login the below error message is displayed:

We're sorry!
We're sorry, but we're currently having problems retrieving order details from the merchant, and cannot process your order. Please return to the merchant's website and try placing your order again.

Here is the debug info from Amazon Seller Central/Error Console:

Error Type:
OrderCalculationCallbacks

Error Code:
ResponseFormatValidationError

Message:
The OrderCalculations webservice returned a response whose format is not valid. Details: Callback response parameter 'order-calculations-response' is not provided.

** If I dsable Enable Callbacks within the module settings it works.

Can you PM the full error message?

Second issue:

I had to comment out line 65 in the file: checkout_by_amazon/checkout_by_amazon_constants.php because it messed up my shopping cart screen (CSS conflict) my question is will this cause any issues within the Amazon module?

Here is the line I commented out:

// define("CBA_STYLE_SHEET", '"https://images-na.ssl-images-amazon.com/images/G/01/cba/styles/one-click.css"'); 

This style sheet is required for enabling one-click checkout popup.

- Amazon Payments