How does it work if I want to choose the exact amount to spend on a brand?

Under "Cash Back Gift Cards," click the box that says "Enter Exact Amount." This will bring you to a keyboard where you'll be able to type in the amount of your choosing.

Keep in mind that some brands will let you choose your amount to the nearest cent while others will require you to buy cards in even-dollar increments. More information can be found on each of the individual brand pages.