Same here. My bill was less then normal too. I signed up right before the billing cycle so I was expected it to be a $1 or $2 more, plus the $18. What I got was a bill for $17 less then normal.
I checked my online account after getting an email stating they did an "adjustment" on my account for $18.. Online it states credit for upgrade fee (they returned the $18 activation fee for upgrading).