WordPress HTTPS (SSL) plugin and insecure form action

I am using WordPress HTTPS (SSL) 3.2.1 and a plugin called Authorize.net for WordPress version 2.3.0. When a user fills out our donation form to donate via Authorize.net, they get a warning popup in some browsers saying the form is insecure even though the page is secured. In Safari the message is:
“This is a non-secure form. This form will be sent in a way that is not secure. Are you sure you want to send it?”
In Firefox the message is:
“Security Warning: Although this page is encrypted, the information you have entered is to be sent over an unencrypted connection and could easily be read by a third party.
Are you sure you want to continue sending this information?”
The only way I can resolve this is by de-selecting the “Force SSL Exclusively” option in WordPress-HTTPS plugin. However, this means that all pages are served as https, and we only want the donation pages secure.
I have contacted the Authorize.net for WordPress developer and they say that the WordPress-https plugin is changing the form submission URL from https to http.
Is there another way of resolving this issue without making the entire site https?
Here is the URL: https://www.newgenerationhaiti.org/donation-now/
Thanks.

https://www.ads-software.com/extend/plugins/wordpress-https/