Taking stock of options

My wife and I went out to eat last night. (It was delicious. Thanks for asking.) Anyway, I was just looking at our receipt. At the bottom, it said:

An optional 18% gratuity will be added to parties of 8 or more.

Doesn't sound very optional to me. It was the Olive Garden. Maybe English isn't its first language.

I tried to take a picture of the receipt, but my little digital camera couldn't focus on it.