Princess does have in-cabin bar set up for pre-purchase through their guest services. The prices are reasonable but a bit more expensive than that. I think the price per liter is more in the $28 range for Stoli.
You can contact them at: 1-800-Princess and select Guest Services.
Make sure to ask for the full liter bottles and NOT the 375 ml. packages. These are available for around $17 and are not really worth it.