Higher mobile service bill than I was offered

I am a current user of Xfinity internet and I sent a request to Xfinity what would be the price if I bring my 2 mobile phones and port them with Xfinity. They offered me plan 20$ for 2 lines per month (two discounts applied). There is a proof of that. But you should not trust Xfinity. As it turned out my bill is 60$ and the following bills will be 40$. 

If you offer special price to customers you must follow your offer. Otherwise you cheat your customers and lie to them.