To my knowledge there is not a way to sell more than one class per transaction or more than one price per course with the core ecommerce functionality of Moodle. I am also not aware of a simple Moodle plug-in that is going to give you these capabilities either. To add this level of ecommerce functionality you will need to use a third party solution.
Some of the more powerful third party solutions are commercial and also require a significant commitment to deploy. I use aMember to control enrollment on my Moodle site and it has the features you are looking for and a lot more. I recently made a post on the aMember site about this integration which you can read here.
There are other third party solutions available so you should consider all your options if you decide to add this level of functionailty to your site.
I was not aware of aMember. This looks VERY interesting. Are you (or anyone else) aware of other similar products. I need to add this type of payment flexibiltiy to my moodle site, but I would like to explore other options that might be out there before picking a solution.