At www.princess.com under FAQs they state that "We kindly request that you do not bring alcoholic beverages (other than wine and/or champagne) onboard for consumption." So if you are going in the day before, I would suggest you save yourselves a bit of money and carry on your own wine for consumption. However, if you would like to purchase wine/liquor/beer to be onboard for you during your trip from Princess, you can do so through 1-800-PRINCESS. The items available for purchase are listed at the following website:
You can download a .PDF form and mail it in or contact them directly and give them the item number and charge it to your credit card. Your room steward will be happy to retrieve these items for you once you check-in.
I think some cruise lines have a deal where you get a discount if you buy several bottles of wine. Princess does not have such a deal. However, their wine prices seem to be reasonable, and I've seen comments that their normal prices are less than other cruise lines.