$5 upgrade

I currently have Internet Starter 10 for $30.98/mo. I got a personalized offer in the mail to upgrade to 300 Mbps for "$5 more per month". When I try to upgrade the price shows $49.99. On the live chat, they just kept referring me to my account xtras, where it comes up $49.99.

The fine print:

Offer expires 1/6/20 and is available to existing residential customers who upgrade their internet service. Offer Upgrades start as low as $5 more per month for internet upgrade. 

Nowhere does it say anything about losing your current price. 

At the top:

XTRAS let you do lots more for a little extra a month.

Does anyone know how I can take advantage of this offer at $5 more per month, as it states?