Paying for someone in the Drive Thru

Last night I was going through Wendy's drive thru and thought that I could take care of number 5 on my list. So I looked in my rearview and there was a small car with two young men. I thought they probably are college kids or teenagers and could use a freebie. So luckily it wasn't busy so they didn't have the whole two window, pay in one and pick up your food in the other or it wouldn't have worked as well. So the lady taking my money took my money for mine and gave me my food. She was in a foul mood, I could tell. I then asked her if I could pay for the car behind me. She was Hispanic and it took a little to get her to understand what I was asking. She was like sure and then rang that up, without any emotion at all. The total was just over 6 dollars so I feel like it wasn't even worth it. They probably ordered off the dollar menu and maybe if they were teenagers had to pool their money to figure out what they could afford. I hurried and drove away. The funny part of the whole situation was that it took so long for me to make my transaction and then to get the lady to understand what I was trying to do that the boys were probably really cussing because they had to wait longer than normal. I drove again sipping my sprite having completed Number 5. Number 5 CHECK.