I figured out the issue that some (or at least 1) person reported in this thread. There is a problematic intval() call in the form generation function. Merchant ID's are generally very large numbers. intval() is either a 32 bit or a 64 bit function, depending on the host. If it's a 32 bit system, it will max out and cap at the wrong value.
For those running into the "oops!" error, please fix the following line by removing intval:
Code:
$form['action'] = 'https://checkout.google.com/api/checkout/v2/checkoutForm/Merchant/' . intval($settings['gmerchantid']);
to this:
Code:
$form['action'] = 'https://checkout.google.com/api/checkout/v2/checkoutForm/Merchant/' . $settings['gmerchantid'];