Ethical dilemma

What would you have done?
Last night, I was two customers behind a couple buying groceries at one of the local chains. The cashier couldn't get their check to run through the machine. She finally had to call the customer service woman. She comes over, tries it herself and says, basically, "Sorry, but we can't take this check."
From behind the middle person, I quietly asked the cashier how much their groceries cost. The bill was about $80. I couldn't do it. Probably like yourself, I've spotted people a few bucks here and there, in a similar situation. But it was too much for me.
Guilty feeling? Yes, some, but also sad for the couple. They had to leave the cart. But also, I have a "save the world" streak in me that needs to be tamped down at times. Because I'm not Bill (or Melinda) Gates or anybody like that.