Doing the Major Drills quests and for some reason I forgot how to restock in bits and bytes. Mostly how to restock at all. Can anyone help me so in the future I don't run into this problem?

It'd be greatly appreciated!

It means you just simply go to that shop he wants you to and buy any item in there (:

I thought restocking meant buying the last of an item available in a shop? Huh, then I did it wrong all this time!

Nope, restocking simply means to buy any item at any time.