What to do when your iPhone 8 Plus back is cracked?
It’s is great condition on the front... will the crack on the back effect anything??
It's difficult to diagnose your issue remotely.
It depends on the crack size. If it is very small, you could hide it with a good back cover case.
I suggest that you, to feel more comfortable, take it to your most convenient Apple Store or Authorized Service provider so that they can offer you all the available options.
Apple - Find Locations - Location Selector
Regards and best of luck.
Giulio