This kind of exploit is a relic of the early-mid 2000s before server-side validation was EZ to do. With the various payment frameworks, APIs, and additional offerings available today, no one is coding these kind of things by hand anymore.
The exact way this worked is based on how the payment system(s) conducted transactions, also due to bad configuration of buttons (think the old PayPal dono buttons). When a transaction went through for a specific item, the price wasn't taken into account, i.e. you could literally pay $0.01. And also this method usually involved instantly-available pay-to-download content like Videos, Audio/Music, Ebooks, etc, where the content needed to be available instantly.
Now-a-days, this is handled by multiple services communicating with each other through backend APIs to verify that the payment was completed, successful, and for the correct amount. At best, you will lose your money.
There are many tools that used to be used to do this. I believe the tool you are referring to was called TamperMonkey or TamperData or similar. You can try Fiddler or even just use inspect element (right click near element).