A Handling Fee is an amount that is added to an attendees invoice.  The fee can be either a fixed amount like $5, or a percentage of the attendees invoice - such as 3%.  A handling fee can be used whatever payment system is being deployed - Paypal, Stripe, Authorizenet, or even checks.    Note: this is an optional feature that helps the host subsidize some of the costs of collecting registrations - but there is not obligation to use one.

You can event create multiple components to the handling fee - such as a $2 fixed compobent and then create a second component that is 1%.

You can also relate a fee - to different conditions.  For example there may be an option dinner - and you might connect a 2% handling fee to just the prices of the dinner.  

To start to go Configure > Reg Types/Options > Hanlding Fees

Here is the result of settting up 2 components to a handling fee.  More could be added no top related to other aspects of what people could sign up for.  A booth fee component for exhibitors for example.

Then, when an attendee registers, their invoice looks as follows:

As you see - the Handling Fee is an extra item on their invoice beow the items they purchased.  And it is displayed separately when looking at reports of Attendees, and on the Revenue Summary.