So I have tried every combination I could think of with authorize.net AIM & ARB active/inactive/Sandbox/Live along with finite vs infinite subscriptions with single and multiple modules. I have disabled all plugins except WPMU and reverted the twenty eleven theme. Turns out that Google XML sitemap plugin was initially preventing me to see the coupon code field. But after adding a couple more subscriptions, the coupon code field has disappeared again.
I followed the flow chart for troubleshooting and still have no solution, but I have some consistent bad results, leading me to believe there is a problem with the code. You can check out my registration page here. The first two subscriptions are real subscriptions I will offer. The last two subscriptions are just to demonstrate the issue. If I have a subscription with 2 finite levels, the pay now works fine. If I have a subscription with just 1 finite level, I get 2 pay buttons and 2 credit card forms. If I have more than 2 finite levels in a subscription, I have no pay button.
Thanks in advance for your help!