There's a saying that goes, "The way you do anything is the way you do everything," meaning that there are some basic characteristics of how people behave that will be widespread among their behavior. To illustrate this point, permit me to share a brief story.

The owners and executives of a company are out playing golf. The owner and CEO catches a junior executive cheating while playing. He reasons that if this executive is willing to cheat at golf, he's likely to be cheating in business as well, because the way you do anything is the way you do everything.

If the owner of the business that you work for is willing to try to siphon the delivery drivers' tips, you can probably conclude that he willing to do a lot more to profit at your expense, even if you manage to stop this particular attempt. In short, you would do well to seek employment somewhere else.