Amazon offers Prime members Reading Adventures: Sofia Pre-1 Boxed Set for $5.99 with free shipping. (Non-members can spend $35 or more to qualify for free shipping; otherwise, it adds $4.98.) That's tied with our mention from two weeks ago, and the lowest total price we could find by $4. It includes ten 16-page books.
Note that this item is temporarily out of stock, but can still be ordered at this price for delivery when available. No estimated ship date is given.