Charge is great for single product sites. There’s no need for any complicated cart setup. Just let your users pay. It’s a simple as that. There’s an advanced action model to let you attach payment information and perform updates after a user has paid too.
Charge only costs $45 (free for non-profits; email them for information) and does require that you have a Stripe account set up to process the payments. The download also includes some sample templates so you can get started quickly.