While I understand what you are saying, but a lot of people don't care about money, because if they did, they would check their bill and in most cases, A LOT of customers do NOT read their bill. Also, if I want something, I don't give a care if it's a couple of dollars more, because that's what I want. So I really don't agree with your logic. You still should have had permission from your manager to do that. Also, what a IRRESPONSIBLE dumb manager that you had to not have you put the coupons with your register z-out(assuming that you had some sort of thing like I did when I worked at a donut shop/diner that you had the register tally up all the sales for the shift). I had to include any coupons, checks, video poker tickets, etc. I also once didn't know because NO ONE TOLD ME some man gave me another video poker ticket from another place of business. Luckily it was only $10 and they made ME responsible for getting that $10 back from the restaurant that had originally had it, which my mom did at the time since I was only 21yrs old still living at my parents house at the time(I am 35yrs old now). My point is, my manager if we were short, they'd take it out of our checks after like $2 or $3 can't remember. Also, you HAD to include any coupons or anything like that.
I truly don't agree that people spent more, because you still underrang items since you didn't include the coupon in your register.
It's possible you could have made more money for the restaurant, but it wasn't YOUR CALL to make since you weren't a manager. If I were a manager, I would have FIRED you ON THE SPOT for doing something without my consent EVEN IF it made more money for the restaurant, it's not up to YOU without permission. If you would have asked permission and you thought it would make more money, then I would have let you do it to see and if it did work, fine, but you would have had MY PERMISSION if I were a manager. I would have fired you for doing something like that. It honestly makes people not spend as much because I don't care if something is $5 more or whatever, I want what I want and don't give a care. What you did it for was that you did it to make more money on your tip, which you did it at the expense of the restaurant to steal from them. You are in the wrong, sorry, you are!!
THOU SHALL NOT STEAL!