If you book shore excursions pre-cruise you will pay for them by credit card at the time of booking. They will appear as a debit on your S&S but there will also be a corresponding credit for the prepayment.
Other than that, any excursions I have booked on board have always gone directly to the Sign & Sail.
All I ask is a tall ship and a star to steer her by; John Masefield
Carnival: Glory 2004, Destiny 2008, Splendor 2009, Freedom 2011, Valor 2012, Dream 2013
Celebrity: Summit 2011
Princess: Ruby 2010, 2014, Caribbean 2013, Coral 2014, Regal 2014, 2016
Sail Windjammer: Mandalay 2017
Star Clippers: Royal Clipper 2015