Checkout by Amazon + Ultimate SEO URLs

Zen-Cart 1.3.9 h

Customers checking out using Checkout by Amazon 1.0 upon "placing order" on amazon are redirected to:

www.oursite.com/checkout_by_amazon.php?string...

Ultimate SEO is adding .html after .php which goes to page not found.

example:

https://www.site.com/checkout_by_amazon.php.html?action=ResetCart&amznPmtsOrderIds=xxxxxxx&amznPmtsReqId=cartId%3A49014&amznPageSource=CartPage&showAmznPmtsTYPopup=1&merchName=ACMElectronics&amznPmtsYALink=https%3A%2F%2Fpayments.amazon.com%2Fsdui%2Fsdui%2Foverview

If i remove .html the URL works correctly.
I am not good with mod rewrites, so any advice is appreciated.
Here is my suspect:

RewriteCond %{REQUEST_FILENAME} !-f [NC]
RewriteCond %{REQUEST_FILENAME} !-d
RewriteRule ^(.*).html$ index.php?main_page=$1&%{QUERY_STRING} [L]

All SEO URL rewriters serve no useful purpose, and in truth have not the slightest effect on your SEO.

As you have discovered, they tend to create more problems than they claim to resolve.

Remove the module... you don't need it at all.

I agree regarding the SEO, however the main purpose of this module was to shorten URL's we email to clients.
Occasionally URLS to products will get broken up because the string is too long.
Also it is more aesthetically pleasing.

Really, i think i am looking for a RewriteCond that would make the URL (https://www.site.com/checkout_by_amazon.php?stirng...) exempt from any rewrite rules.

CEON URI module is what you should be using then... It is designed to achieve the purpose you intend.