He remembers that he wanted to get sugar and caffeine, in any form. He doesn't really care what form it is in. If he had remembered, he would have gotten sugar--in the form of a doughnut--and caffeine--in the form of "Red Bull." Red Bull is the brand name of a so-called "energy drink." It is carbonated, comes in small cans, and contains a lot of caffeine (and some other ingredients, such as taurine and B vitamins, that may or may not have a stimulating effect). If he'd stopped in a place and it didn't have doughnuts, he might have bought a candy bar. If it didn't have Red Bull, he might have bought coffee. "Discharge" is a very odd word choice here, but a Google Books search confirms that it's the word the author chose.

Red Bull is a brand of drink which contains a lot of "sugar and caffeine"

Hello. I'd say that the sentence you are asking about only explains the reason why he wanted to buy them - to get some sugar from donuts and to get some caffeine from Red Bull that would help him to manage his way to Rapid City. Given no further context it is hard to say whether he bought them or not after all, but if nothing that could break his intention to buy the products had happened, he pretty much bought them.