Some excerpts from Carnival site where you can find more information on Sail & Sign.
Acceptable Cards
Visa Card and Visa Gift Card
MasterCard and MasterCard Gift Card
An ATM/Debit Cashing Card may be used as long as it is a registered MasterCard or Visa Card.
The Discover Card and Discover Gift Card
American Express Card and the Carnival Corporation Reward Card from American Express
Diner's Club and Diner's Club Gift Card
JCB Card only
Note: Gift Cards may also be referred to as 'secured' or 'pre-loaded' credit cards or 'Travel Funds Card'.
Unacceptable Cards
Gift Cards and Travel Funds Cards issued by: American Express and Optima
Keychain (mini) credit cards
The American Express 'Persona Select'
ATM/Debit Cashing Card other than those registered by MasterCard or Visa Card.
Cash Deposits: If guests use cash for their Sail & Sign deposit, any refundable overages greater than $10 USD will be refunded via check mailed within 7 days after the cruise to the guest's home address on file. Overages less than $10 USD will be donated to St. Jude's Children's Hospital® (excluding the Carnival Spirit® and Carnival Splendor® in Australia). This will be communicated on board during the Debarkation Talk and posted in the daily 'Fun Times'. If a guest wishes to have the full overage in cash, they may retrieve it from one of the Sail & Sign Kiosks or the Guest Services Desk. Refund checks will be reconciled by our shore side Refund Accounting team and distributed via PNC Bank (JPM Chase Bank for Carnival Spirit® and Carnival Splendor® Australia guests).