Because our website does not have an SSL, we are looking to use SIM to process payments. We'er on a UNIX platform and utilizing PHP. However, it appears that you have to pre-set the amount that the person is being charged in the code. I was reading a post on this site:

http://community.developer.authorize.net/t5/Integration-and-Testing/SIM-integration-help/m-p/1678/hi...

And the Developer stated that:

"However, you should know that SIM is not set up to allow the customer to enter the payment amount.  That needs to be determined before you ever redirect to the SIM payment form.  In fact it needs to be fixed before you even render the form fields which are submitted to Authorize.Net's SIM form.  There are ways to make this happen, but how depends on the language you are working in".

Our users will each have a different amount for each invoice that they wish to pay online. They need to have the ability to enter in the amount of the invoice. However, If I am understanding this correctly, there is not a way to do this with SIM. Am I correct? Also, if there is no way to do this with SIM, then it appears the only solution we have is to obtain an SSL certificate and look for another method of integration.

I would appreciate any help anyone may be able to offer. I will be completely honest and state that  this is truly out of my area of expertise.