Review Devalued Sundaes

I bought sundaes from Mc Donald's Halang in Calamba City (Laguna) for myself, a PWD, and my Mom,a senior citizen.
I waa told that thete was a limit to the number of sundaes which will be given a discount. After a little argument, she changed her statement and said that the limit was for the senior citizen. I did not argue any further because I was tired from work and wanted to get home. When we opened our sundaes at home, we were shocked to find that the 20% discount would give us 50% less of the usual Mc Do sundae. (picture attached)
Is that how Mc Donald's treats people accorded such discount privileges by law?
I am a government physician who pays my taxes well.
I hope you look into this irregularity.
Thank you.
We love McDonald's sundae. This is such a disappointment.
Please fix this disgraceful service.