We were there in 2018 I think. We thought spending $450 was a lot.
RC realized, likely due to cabanas on Coco Cay, that they were woefully underpriced.
That's not what RC's permission letter says though.
https://www.royalcaribbean.com/content/dam/royal/resources/pdf/minor-traveling-without-parent-or-guardian-form.pdf
The OP is asking about a permission letter for an 18 yr old, which is not needed, despite the 18 yr old not being old enough to be the oldest on a reservation.
That just says the 18 yr old can't sail alone. It doesn't say the 18 yr old needs permission from his parents.
On the consent form RC provides, minor is 0-17 yrs old