Just spent about 5 minutes going back and forth trying to extend my GB premium for the sale price until I figured out the text above the submit button was a link to the page I needed to use. If you click the submit button you just get the error about already being on yearly renewals.
Suggestion: either make the submit button take you to the correct page instead of throwing the error or, if that is a PITA change the style of the link to make it way more obvious (e.g. make it a button or something). I'm using Firefox 68.0.2 on Windows 10.