There is no insurance for "a customer claims that your product cost them millions". You will come across some clients who claim that regardless of what happened

There is no insurance for "a customer claims that your product cost them millions". You will come across some clients who claim that regardless of what happened. However, you could buy something called "Errors and Omissions" (E&O) which is an insurance for disasters in case something happens to customer data etc due to your fault. I have used a broker called techinsurance which seem to be decent: https://techinsurance.com https://techinsurance.com also, hire a lawyer to get solid T&C in place and make sure that customers are aware of any changes in T&C. Keep a good relationship with the lawyer and make sure that any potential legal challenges are discussed with the lawyer. It will cost you a bit but provides peace of mind. If a client is threatening to sue, get the lawyer involved.