Which only works if your iPad battery doesn't meet Apple's own 80% threshold, using their own in-house diagnostic tools. It's not like the iPhone, where Apple replaces the battery for you so long as you are willing to pay, no questions asked.$99 is Apple's out-of-warranty battery service.
That's what happened with my 9.7" iPad Pro. Battery life was getting extremely bad after iOS 12, third party battery apps pegged battery health at around 80%, apple store rated it at 94%, and so I was out of luck.
The simple reason for this is that Apple replaces the iPad altogether, rather than just the battery, so it's definitely costing them more than a simple $99 in this regard.