What is the difference between Authorize.net SIM and AIM

The main differences between Authorize.net SIM and AIM are: 1) how the payment gateway is integrated into your website, and 2) what the checkout experience is like for your audience. 

SIM: Your customers leave your site to go to a secure page to complete their order. They are then sent back to your site for a thank you or whatever else you want to do. This is sometimes seen as "less professional" because it redirects customers away from your website and then back.

AIM: We use Authorize.net's API to communicate between servers behind the scenes to process a sale. Your customers never leave your site and the whole process appears to be transparent. You need to provide the SSL cert (which we provide for you) and all code to integrate the API. This is seen as "more professional" because people stay on your website to process their payment.