Promised one price online chat then given a more expensive price

I went online to check about changing some of my services.  I have been a customer for over 20 years.  I was told I could switch my internet to 1 Gig speed and pay $80 a month plus tax for 5 years.  And I could receive a $10 per month discount for direct pay.  I went into the store to finalize this since I needed to drop off a box and pick up a gateway. At the store I was told that the person I was chatting online with from xfinity was incorrect and that it would be $110 per month plus tax.   I have the offer from the online chat in print and showed it to the representative at the xfinity store.  His response was he cannot honor that.  The online person was incorrect.  I feel it should still be honored.  It is not my fault they online person offered me something that the store said couldn't be honored.

I have also emailed Tom Karinshak's office concerning this.  I hope someone can give me some help.