Question: My card transaction exceeded the sanctioned limit and an overlimit fee appeared. I never opted in. Is that valid?
Answer: No. Using a credit card beyond the sanctioned limit requires prior explicit consent from the cardholder. Without explicit consent, the issuer cannot provide the overlimit facility or levy overlimit charges.

Supporting evidence:
title: rbi.org.in/BS_ViewMasDirections.aspx
quote: Usage of a credit card beyond the sanctioned credit limit (i.e., overlimit) requires prior explicit consent of the cardholder, as a fraud minimisation mechanism Unless explicit consent has been obtained from the cardholder for the overlimit facility, no overlimit can either be provided or overlimit charges be levied
context: o the cardholder to carry out the modification? Response: The cardholder shall be provided option to choose any date as the starting or closing day of the billing cycle at least once. Further, card-issuers may provide the option to modify the billing cycle through multiple channels such as helpline, dedicated e-mail-id, Interactive Voice Response (IVR), internet banking, mobile-application and any other modes. Query 10. Can a card-issuer allow a credit card to be used beyond the sanctioned credit limit and charge overlimit fees? Response: Usage of a credit card beyond the sanctioned credit limit (i.e., overlimit) requires prior explicit consent of the cardholder, as a fraud minimisation mechanism. Further, a cardholder shall be given an option to enable or disable the option of overlimit through transaction control mechanism available on card-issuer’s platforms such as internet, mobile banking, or any other suitable medium. Unless explicit consent has been obtained from the cardholder for the overlimit facility, no overlimit can either be provided or overlimit charges be levied. Query 11.

Whether interest, taxes or any other charges can be considered for computing a cardholder's credit limit usage for the purpose of levying overlimit charges? Response: Interest or any charges/fees on the credit card shall not be factored in for the purpose of computing a cardholder’s credit limit usage and levying overlimit charges. Query 12. Whether the liability towards credit card dues lies only with the principal cardholder? Response: Yes, the liability for dues shall rest solely with the principal cardholder and not with the add-on cardholders. However, settlement of dues towards international credit card shall also be governed as per FEMA regulations. Further, the responsibility for making payments in case of business credit cards shall be governed by the terms and conditions agreed upon. Query 13. Are card-issuers required to provide an insurance cover on debit and credit cards? Response: The Reserve Bank has not prescribed any requirement for insurance cover on debit or credit cards. However, in case a card-issuer or a card payment network provides an insurance cover, complimentary or chargeable (with the consent of the cardholder), the card-issuer shall ensure that the relevant nomination details are recorded by the Insurance Company and the availability of insurance is included, along with other information, in every statement. The information shall also include