Accepted promo phone monthly $$, but inputted wrong by support - now charged full rate

Support sold me on a $400/device promo last November and I upgrade 2 devices for the kids as they had really old ones.  Doing a chat with support just now, the support that "helped" me added both phones at the standard rate! - I was offered a $30 credit - denied that.  I unfortunately havent looked at the billing until now but regardless, xfinity messed up and needs to fix it!

Its a good thing I save all conversations

Long chat but here is the main idea:

Thank you for waiting, the system is allowing me to do one line at a time, I tried to add Julias new iPhone 16 with Hannah but it only lets me do one at a time. it shows here it would be $76.76 down today this includes taxes and fees for Julia's new iPhone 16 pink 128GB and $13.75/mo ($400 credit applied for 24/mo) would you like me to text or email the consent to you?

How can this not be fixed?????  XFINIITY do the right thing as your side messed up.