How to add a simple online payment transaction?
I agree that Commerce Kickstart is too advanced for what you need. You'd end up spending a lot of time configuring "off" a lot of stuff just to give your users a simple checkout. 

I've setup several simple custom payment solutions for Drupal sites using Webform and Stripe.com. 

Here's an example of a Stripe/Webform component: https://drupal.org/sandbox/jlyon/1472346

Basically you'd build out the component that captures their CC info and submits it to Stripe via javascript, then run the charge on the server side using the token that Stripe gives you. This gets you out of the bulk of PCI compliance (you just need to have SSL enabled and ensure that you don't let any CC info be sent to your server). Since Webform is easy to use and you can add whatever form elements you'd like, you can quickly capture the right customer info and set the "product" or dollar amount in a dropdown field, then run the charge in a _validate() function.

Not quite plug and play but also not a lot of development time to get a solid working solution. 

Edit: You mentioned PayPal – you could provide that as an alternate purchase opition and use a simple paypal button to send them on their merry way; also https://drupal.org/project/webform_paypal

Edit 2: After thinking about Ryan's comment a bit, I would suggest you first grab Commerce module and tinker with it for a while. You may find that it gets you what you need. I have really enjoyed implementing some custom, lean payment systems using Stripe where a traditional "checkout" workflow wasn't appropriate, but there's no need to reinvent the wheel if it fits.