You're right, I tried calling and it took them 15 minutes to find the excursion and another 10 minutes to find the hidden note with the age restriction. Their on-board policy is that you may cancel onboard for OBC provided that the excursion is at least 48 hours away. I board on Sunday and Ocean Day is Monday so I don't see that as being an option. They may very well make an exception at the shore excursions desk but without any written policy in advance, I guess I'll just have to cancel and wait 1-2 months for a refund.
This is my first cruise with MSC. I'm sure the trip itself will be fine but so far, I'm not impressed.