What is the best shopping cart to use with WordPress

I am looking at setting up a wordpress website integrated with a shopping cart whereby the client wants to sell first aid courses. Basically these courses do not require shipping methods and will require attributes to enable the viewer to add a number of candidates to the course. Each candidate name he enters it will populate the price per course per candidate.

Can anyone recommend a shopping cart facility that will accomodate this or any advice would be appreciated.

Thanks in advance