Is there a guarantee the checkout fix won’t break my live store?

Yes. Two safety nets: (1) every fix is reproduced and verified on staging with a real test order before it goes near production, and (2) every job carries a fix warranty, 7 days on a Quick Fix, 14 days on a Bug-Fix Sprint, so if the same bug comes back, we fix it again at no charge. For checkout emergencies that can’t wait, see urgent Magento bug fixing or the emergency e-commerce bug fixing hub.