Most sites promote free shipping offer on cart page like this (red arrow in screenshot below)–
This is a 6, on a scale of 10. This is how Amazon.com promotes their free shipping–
In both cases effective shipping price is $0. But Amazon’s approach is more effective because they have an action element. Amazon first shows the $6.98 shipping fee, and then waives it for me. I’d give this tactic a 9, on a scale of 10.