Why can't I choose a new plan that is being offered to me?

My contract is up in a couple of weeks, so I went to choose a new plan.  There was a decent offer that had almost all the channels I currently have and offered a speed bump.  It comes with a 12 month commitment.  Fine, sounds good.  I go through to change to it, but it will not allow me to complete the checkout without adding an additional TV box.  Even though I have selected the one I already have (it comes at a $5/month charge) it will only allow me to proceed if I add the additional box.  I spent what seemed like an hour with the "Xfinity Advisor" in chat, and he said he couldn't find the plan, but did find a more expensive plan with a lower internet speed.  That doesn't sound good.  He said I should get the extra box and return it and the $9.95 fee would be removed.  That's not acceptable to me.  The final suggestion was to call and talk to someone, but to be honest, I don't want to spend another hour talking to someone and get nowhere.  Why does Xfinity make things so difficult?

It doesn't look like this forum allows me to insert an image, but here is a link to what's happening to me: https://imgur.com/a/DhZfxqJ

Please help, anyone!