For merchants in the US, Mastercard credit transactions are subject to additional rules for gratuity:

• If the card data source is Chip/PIN, Contactless, or Card-not-present, any gratuity must be included in the authorization request. A gratuity must not be added to the authorized amount.
  Exception: Restaurants (MCC 5812, 5814) have a 20% gratuity tolerance for Mastercard transactions; if the gratuity exceeds 20 percent of the authorized amount, these merchants may request an additional or incremental authorization for the amount in excess of the authorized amount.
• If the card data source is a signature-based magnetic stripe or Chip (without PIN), a gratuity may be added after authorization is obtained; a 20% tolerance applies to the gratuity. If the gratuity exceeds 20 percent of the authorized amount, then the Merchant may request an additional or incremental authorization for the amount in excess of the authorized amount.

For any request, if the authorization response indicates a Partial Approval (RspCode = 10), gratuity may not be added.