Why would you want to force the people who want to hire you to have to go through another step just to process an order? If people click the visible, hard-to-miss, clearly explained green button to purchase your services, they probably know that they want to work with you, and have already decided to hire you.
If they choose to hire you, work with them. Provide them with the service they hired you for.
I find it silly when some buyers claim that, “I ordered by mistake.” How do you click a big green button that says “buy now” (or, whatever the specific button they click says), and then confirm an order… by accident?
You can usually avoid most cancellation situations by working with the buyer to provide them with services that do not need to be cancelled. Yes, there are rare and valid circumstances in which a cancellation might be the best course of action… but those can be addressed as they happen.
A “do I want to work with this buyer” pre-sale, pre-approval button is generally a pointless, and discriminating concept. It is probably best to work with each situation on a per-situation basis. After all, the entire point of being a freelance seller here on Fiverr is to work with your buyers to fulfill their needs. That cannot be accomplished by picking and choosing who you work with, AFTER someone has already chosen to hire you.