McDonalds coupons

This coupon will always remind me when Mc Donalds (my favorite fastfood) sort of tricked me.
OK let me tell you a short story (it may bore you but nevertheless I will write it) :

Last night around 8pm, I dropped by McDonalds Fuente to buy myself a meal. I reach for my wallet in my bag and saw this coupon telling me that I will get P10 off from my meal. "Hmmm... sounds like a great deal.",I thought. So approach the counter handing in this coupon.
The crew told me that I only pay P60. "Hmm.. not bad", I thought. She then asked me if I needed a drink. I ordered a drink and then the total amounted to P79. OK.. I scanned through the display and saw that meal A only cost P70. So now, I feel like I'm being punished for having the stupid coupon. Instead of paying less I get to pay more. "That's not fair. That's ilad.", I thought aloud. The manager reasoned to me as to why I HAVE TO PAY MORE. She said it was based on single orders .I PAY LESS P10 compared to having single orders. So now, I'm asking what's the use of coupons then? I told the manager, irregardless of the fact that her reasoning is right (technically), I still think that it should not be the case. The fact that I was given a discount coupon that does not give a discount is a betrayal of trust. Ergo, don't trust the coupon. It was stupid to have it. It just doesn't make sense.

P.S.
This is not a rant against McDonalds. Nor is it a complain because McDonalds gave me the opportunity to pay P9 more than I expected. I love eating Mcdonalds but the coupon was just terrible technique. It doesn't give customer advantage it just created a false sense of advantage.

I was fooled.