Know-How Exchange : E-Marketing

What shopping cart offers an affordable yet secure, controllable way to sell single and multiple licenses to digital files (PDF publications). (My client wants to buy the license to a shopping cart software and does not pay a monthly usage/hosting fee.) The audience is government and non-profit boards and they would be purchasing one license per board or staff member. Would it be smarter to release content as e-books rather than PDFs?
Thank you