Sorry Out of Cash

So I was at the Commissary today, and I did something that I have not done before...I stiffed the bagger on her tip. I feel really bad, especially since I know they don't get paid by the hour and work solely for tips. The girl was really nice, and she bagged the stuff well. To top it off, I couldn't remember where I parked my car, and she literally walked from one end of the parking lot to the other helping me look for it. So why the stiff? Trust me it was not intentional; I thought I had some cash in the glove box (apparently my husband took the gas money and forgot to tell me...good thing I didn't need gas). I promised her I would go to the ATM and get her some money. But here's the twist and why I'm not sure if the guilt trip is warranted or not. The guy waiting for my parking space handed the girl a $5 bill, so should I still feel bad? I did attempt to go to the ATM, but while I was trying to remember my PIN number (which I still can't remember...dang baby eating my brain), a police officer came over and almost gave me a ticket b/c I left the car running (with Nevaeh in it) in a Fire Lane. I explained I just needed to go to the ATM, but he told me that it was a serious offense and that I needed to park the car. I decided to leave instead, but now I'm feeling really guilty that I didn't give the girl a tip. Am I overreacting or should I go back and slip the girl the $5 she deserves? What do you think?