Maybe call the ferry at a different time and see if its the same results.I know I read all this but the booking guy at the ferry told my wife that it is the 24 hour rule.
If you plan on taking a taxi at the ferry terminal be sure you have some cash.
There are no machine bank tellers at the ferry terminal and the PR taxis there will not take a credit card and will not take you to the nearest bank so you can withdraw cash to pay them.
Ask me how I know.