{ Sticky Bar } 400% & Keyboard Navigation

Has anyone worked with a sticky bar that grows when a user makes a
selection from the page/screen? We have a page where a user selects items
to purchase and there is a sticky bar at the bottom that collects the
selection like a cart. A person can also adjust the quantity of the items
in that bar.

I have concerns around how it overtakes the page when increasing the % as
well as line and character spacing. With respect to screen readers, I think
some instructions on how this page works would be needed. (the product
expands to display descriptions).

In the meeting with the designers the requirement was that the sticky bar
must be on the same page as the product list. I wondered if giving the user
to option to expand and collapse the bar would help. Although having to
navigate between the product and the sticky bar itself seems like a chore
as well. i suggested perhaps giving the user the option to have the cart as
a sticky bar or on a separate page but that was shot down.

Any thoughts? Hope this doesn't sound too abstract