It has problems with the thickbox style as others have noted unless it is commented out then it doesn't look right. Or use the legacy call with thick borders and it will work. It seems no efforts have been made to resolve this in the forums.
Next it does not provide a receipt by either print or email. Perhaps it was not meant to which is fine but that is something most any retail type store is going to need to provide to their customer outside of non-profits. So that was the breaker for me and I cannot use it.
It has potential to be a great plugin and I donate to all the developers whose plugins I use but this one is just not there for me yet. I will pay for a plugin that provides these features.