I got one enrolled in the scheme today, Amazon decide which of your products qualifies and then auto enrolls them! you only get notified by email after it has gone live. Amazon puts up a 5% discount (out of their fees I assume) and you have the option of increasing the discount at your cost.

If they're products not easy to restock it might be worth unenrolling them from the scheme although I suspect some customers will cancel themselves after they've enjoyed the first discount. I suspect cancelling a customers subscription due to lack of stock will work against you for account health so I'd much rather be given the option to enroll rather than auto enrollment. I also worry subsequent deliveries that the customer has forgotten about will lead to a high rate of returns.

The email is titled "Your Most Recent Subscribe & Save Product Enrolments" and starts:

Dear Seller,

Congratulations – one or several of your FBA products recently became eligible for participation in Subscribe & Save! The newly eligible item(s) have been automatically enrolled at your preferred default enrolment discount of 0%, with Amazon funding an additional 5% discount for tiered deliveries when customers receive five or more items in one delivery.

The email says "one or several of your FBA products..." and as I only do FBM I could have easily stopped reading at that point but it affects FBM products too.

At the bottom of the email there is a list of the product/s Amazon has auto enrolled for you.