Alrighty, found the actual fine print! God I hate dealing with the phone company.
“Charge cap: You’ll be charged a maximum of 10 daily fees per line, per monthly bill cycle. After 10 daily fees on a line, that line may continue to use IDP through the end of the bill cycle at no additional charge” Link
$100 it is.
This happened to me exactly while in Ireland. I went the first few days with phone in airplane mode and only using WiFi, but I had to use What's App with a rental host and we were in the country driving. We later had to call a few places to book or change activities we were doing -- anyway, I hit the 10 days and have a $100 charge (Verizon). Somehow my DH has cellular on 1 day and he got hit for $10, but that was a mistake.
We downloaded all the google maps ahead of time and used those offline a number of days. No issues.
I do recommend leaving your phone in airplane mode and just using wifi if at all possible. Once I 'broke the seal' of spending the $10 I tended to use my phone more often (though I did delete some social media off my phone before we left -- don't plan to add it back, it was fine w/o it).