This strategy is not quite as targeted as including an optin form on postcards with shipments, but it can still work.
You can send postcards to lists of people that you have purchased or in targeted geographic areas. These postcards should focus on the benefits of signing up for your list and visiting your website. This can be a very time-consuming and costly experience, so make sure you're going to see a return on your investment by doing a small test first.
But focus on your ROI, not your conversions. Your conversion rate may be very low. Abysmal, in fact. But your ROI may be very high. But in order to know your ROI, you need to know the lifetime value of a subscriber. If, for example, you pull in only 5 subscribers from a postcard-mailing campaign, but each is–on average–worth $100, then anything you spend under $500 for that campaign in a net gain.