How to make the cart charge an specific value for one or more products in it?

Hi, I'm using Marketpress integrated with Appointment + and i need an specific function to make the site work as i want.

I'll be chargin the users $5 for making appointments in the site, and does not matter if the user makes 1 or more appointments in the same session. I'll charge a fixed value for it.

A+ developer suggested that i use a Discount in the marketpress to make the cart reach the price i need.
Can anybody help me with that?

Today for each appointment is added $5 to the cart, and the function would be simple. check IF there is a any product in the cart. IF not, the regular price is added. If there is one or more, the price of the second, third, fourth, etc should be 0.

Any ideias?

Regards,
Diego