Official comment
Hi Scott - the pricing information is in Account > Printing Credits. Post cards are available for US based addresses only, and are priced based on the total "printing credits" you buy at one time. If you deposit $10 to $100 in print credits, the post cards are .97 each. Click your profile icon at the top right, then Account. Here is the account screen where you can find the Print Credits link:

When you click that link the following table pops up:

While the best pricing is available if you buy more print credits keep in mind the credits are non-refundable. They will never expire, but if you simply stop sending post cards or cancel your account they will not be refunded.