Credit cards, Paypal etc all have fees and charges associated with them. If you would rather your customers paid those charges, then instal this addon. It works by calculating a percentage of the order base price which it then adds to the order as a "convenience fee" or however you would like to describe it. Alternatively, you can elect to add a fixed mount instead.
If you have multiple payment methods, then you can assign a different surcharge for each one.
California law prohiits credit card fees:
Therefore you should seek legal advice before using this addon in California.