How Do You Explain A One-Off Onboarding Or Setup Fee To A New Client?

In episode 179 of our weekly Hump Day Hangouts, one participant asked how to explain a one-off onboarding or setup fee to a new client.

The exact question was:

Hi Guys,
When you are talking to a potential new client do you put in the proposal a one-off onboarding or setup fee, if so, how do you explain what the fee is for, and why do you charge the fee?

Thanks in advance