I don't know if I have to reup my account to get support on this or not. This month I wondered why sales were off and somehow upgrading the plugin to the newest version from the previous version broke the checkout on all browsers except for firefox using the modern checkout theme. Instead of going off on a lengthy description, I'll just make it easy and screencast it, here's a youtube link with me describing/showing the problem.
I unfortunately cannot leave the live site like that, because nobody can buy services and I can't have that. I have it temporarily on the step template, even though I show something about that that isn't working at the end of this as well.
In a nutshell I can only checkout with firefox when using modern, after the domain configuration with chrome and internet explorer, the progress bar just keeps moving and it never finishes. It prevents the checkout from continuing further.
Then with the step theme when you get to the final page where you enter payment information, if you click the existing customer tab you get a blank tab with nothing in it. You can switch between new customer and existing customer due to this.
Screencast of me demonstrating both:
I don't know how to show you the live product though, the add product page with the step theme is what's active at:
But I can't activate the modern template for any length of time because it prohibits people from being able to checkout.