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The algorithm is actually quite simple (apart from the sometimes noted exceptions).
1. Each store has a set of postcodes they deliver to at their listed price.
2. Each store has it's own price for each wine listed for delivery, even if they don't stock it.
3. When you order for delivery, no matter what your store setting for ordering, the lowest price at stores in your delivery area is allowed, but this may be much higher than your selected store.
4. Your delivery, once the order is accepted as above, can come from any store Australia-wide that has stock, no matter what their listed price. (There appears to be limitations on this per store, hence my ability to pick another store after the one I first used said no more stock, not that they had any in the first place, yet the price for delivery was honoured in the delivery area for the store with no more stock when I bought from a store with the low price in another area. Follow that?) In fact I tried an order from a high-priced store for delivery to my address within the delivery area of a low-priced store (with no available allocation) and the price reduced nicely in the cart at checkout.
Update: There is one "gotcha" in the above, last bit of #4: The store with the low price has to show stock available (even if they have no allocation left), when they run out of allocation and the system (or someone) eventually updates and marks the item as oos, the trick no longer works.