This site's own guarantee section states this directly: retain your order confirmation and bottles until the process is completed.
A refund request is easier to process with both documentation and the physical bottles on hand — worth saving your order confirmation email and not discarding empty or unused bottles until any refund process fully completes.
Pair this tip with marking your refund deadline as soon as your order arrives, so you have the documentation, the bottles, and the timeline tracked together from day one.