What is the exact model number(s)? And does Woot guarantee the battery health is at least a certain percentage?
Hi there. Model numbers vary by color/capacity. Which one are you interested in.
As to the battery:
I just received my iphone XS today, and the battery health is at 79%, which Apple classifies as “significantly degraded”, and definitely NOT at the “minimum 85% capacity” as listed.
What now? Do I have to return the phone to get this rectified?
Sorry for the issues. 
If you haven’t yet, please contact CS.
To contact CS:
From a browser, use the Woot! Customer Service form.
In the Woot! App, choose Account from the bottom navigation and then Support.
Note: Woot! Customer Service replies go to the email address on your Woot! account, not your Amazon login email if used/different.
(Please note: I don’t work for Woot, I just volunteer to help out here on the forums.)