I got this off of Carnival web: Link
For online cancellations that occur outside the cut-off window, 3 days prior to sailing, a full refund will automatically be posted to the credit card that was used to pay for the shore excursion order, regardless of whether or not the person who purchased the shore excursion is sailing. Please allow 7-10 business days for the credit to post to the your account. No changes can be made within the cut-off window. If you need to make changes after the cut-off date, you must wait until sailing and see the Shore Excursion Manager onboard. Once onboard, any refunds will be made to the Sail and Sign account of the Shore Excursion order contact personís Sail & Sign account, not the original credit card used to secure the online order. This applies even in the event that shore excursions were purchased as a gift by someone who is not sailing.
In the event of a cruise booking cancellation, within the cut-off window, a full refund will automatically be posted to the original credit card used to pay for the shore excursions order. If the designated contact person on a shore excursion order must cancel from their cruise booking and the remaining guests who are sailing would like to retain the shore excursion order, a new contact person needs to be assigned. If a designated contact person cancels and does not reassign the responsibility, the associated shore excursion orders will automatically cancel and be refunded.
Once onboard, cancellations are subject to a 25% cancellation charge. However, within 24 hours of arrival in port, all shore excursion orders are non-refundable. A refund, less any applicable cancellation charges, will be applied to the designated contact personís sail and sign account. Carnival will only offer shipboard credit for shore excursion order refunds, but note any credit not utilized by the end of your cruise will be refunded via check to the orderís designated contact person on the morning of debarkation.