What online payment system is the safest and best for very small transaction amounts?

For something lightweight you should take a close look at paypal and google checkout:

https://checkout.google.com/sell
http://www.paypal.com

Both allow you to avoid having to touch or process people’s credit card information. Many companies offer both choices to their customers giving the customer the choice to use whatever service they prefer.

You’ll find a wealth of information on both of these services and how they compare here: http://www.google.com/search?q=google+checkout+vs+paypal