Woof. I was considering kicking the tires on Netlify, but they are officially out of consideration now.
You need to manually upgrade.
Unless you need more than the Free-Tier Vercel should be fine.
"Should be fine" is exactly what is not the case here. Better check your Vercel terms again.
From every Vercel document that I can read though is that when you exceed the limits of free tier they are just locked for 30 days unless you upgrade to Pro.
So unless I am mistaken this cannot happen on Vercel.
They removed this call in the meantime [0].