Hey Alexandre,
Thanks for taking the time to share your insight! Whilst I agree that it’s never nice to get an unexpected bill, I do believe that we make it clear to customers when charges can be incurred.
Firstly, the site and the account which you referenced in your Helpdesk ticket to us is on the Starter plan (we have purposefully never called our plans Free, because they’re not). We do offer an Open Source plan that has largely the same provisions as our Pro plan – though even these plans may be subject to additional charges.
Specifically, you received 3 charges for $7 due to additional build minutes being used. I believe that our pricing page makes it very clear that the Starter plan includes 300 build minutes per month. When this is exceeded, an extra 500 build minutes are made available at the cost of $7.
This information can be viewed on your billing pane: Netlify App
An insight in to your build minutes usage can also be seen within the UI: Netlify App
You should have received emails making you aware that your usage was on the up and you were set to exceed the limits of the Starter plan.
We have a great write-up from our Director of Support on how to optimise your build times, too. However, it looks like you found this or you were able to pause builds yourself! 