Apple's policy has always been to replace.
A couple years ago they started equipping their stores to repair because that was more economical for them. But they determine ultimately whether a device will be repaired or replaced as that is their discretion.
It does not mean that if you send in for a new battery and they are charging you for that, that they will then give you a replacement and charge you for a replacement. They would replace, but only charge you for the battery.
I will give you an example, although it's not battery related…
My local Apple store has the ability to repair devices. So, I went in to have a cracked screen replaced. This was around the time of iOS 9.0 being released. They had me restore my phone. I waited an hour. They came back and told me that they could not get my device to boot, so they were replacing it entirely. That was their discretion. But all they charged me for was the screen replacement - because that's all I was there for.
In all probability OP's battery will simply be replaced, but I am simply covering the range of what could happen.