I hate WooCommerce for so many reasons, so it's extremely frustrating that we're forced to use WooCommerce if we want to directly integrate payment for user-submitted content. It appears that LearnDash works as well, but I'm getting ready to stop using them because I'm using another plugin now. What needs to be done to hook in another e-commerce plugin so that Toolset can access their product lists? Clearly it's possible, so if I can add a little extra code somewhere, I will happily do it!
Think of the workflow scenarios that you need to handle: between the Forms API and the APIs of your ecommerce solution and WordPress itself it should be possible to come up with a solution.