Should the back button in a stepper be prominent?

I have a checkout page on my application that has two buttons, one to review the order, and one that goes back to the shopping cart.

My product owner feels that the Back to Cart button needs to be more prominent, despite there being multiple ways to get back to the cart (the link in the header, browser back button, or the Back to Cart button). It's also a secondary action, so it really shouldn't be prominent in my opinion. In early testing of this, nobody had trouble getting back to the shopping cart, regardless of how they chose to get there.

Is there a reason to make this stand out more, and is there a way to make it "more prominent" without pulling focus from the primary action?