Xfinity Plan

I was previously on a plan for 1 year.  This ended about a month ago.   Without warning Xfinity billed me for 90$/month, I switched to Tmobile for a few days and then saw Xfinity had a 40$/month for 5 years even for existing customers.  So  I signed up for the 40$/month for 5 years which was valid for existing customers (or so I thought). I received a 90$ bill and called customer service and they credited me down to 40$. However, now I am realizing why last month's bill is $90. It seems like I am on the 90$/month bill instead of the 40$/month plan for 5 years which I signed up for. Can someone please help me with this.