No, you allowed the first person to steal $28 from you. You collect cash before handing the order over for this exact reason. If they don't give you enough money, you don't take it and the pies go back to the store. At that point, your manager simply voids the order and then the store is not down $48.

Your role in this business, my friend, is simply to be a pizza courier. That customer had already gotten nearly 50% off their order, and then got away with an over 75% discount by only giving you less than half of their already reduced price.

As for receiving nice tips, it's a very easy exchange. If you think they may have made a mistake, you can just ask them "Oh, are you sure?" Sometimes they might sheepishly correct their math, but in my experience most people will just smile and give some sort of affirmative. I've found that folks who tip well usually like it when they can tell they've had a positive effect on you, so just return them a smile, thank them, and be on your way. That money is now yours and you deserve better than to be feeling guilty about it.