What's the difference between a Double Bottle Subscription and a Double-Up?
One Bottle Subscription means you receive one bottle per month ongoing as your standing order plus $15 flat fee postage. You have to actively Double Up if you'd like two bottles in a particular month.
Two Bottle Subscription means you receive two bottles per month ongoing as your standing order plus $15 flat fee postage. If you Double Up you get four bottles for that month.
The key difference however is that Members on a Two Bottle Subscription are always guaranteed their second bottle, whereas sometimes Members can is out on Double Up requests if a whisky is wildly popular. See How do I change my subscription? (single to double, or double to single).