Whereas a brick and mortar store will have business card printing done as part of its marketing mix, a software development company would tend to consider only the online options. Whatever the nature of your business may be, both offline and online advertising options must be considered if you want to maximize the exposure of your products.
To facilitate the distribution of your postcard printing project, you will need a mailing list for the task. Most start-up companies are constrained to purchase these lists since they have nothing to begin with. In going by this route, you might as well spend more on your mailing list and obtain a more detailed list. This way, you can identify your target market early on and sell only to them without blindly reaching out to everyone and draining your advertising budget.
All major websites are offering some sort of advertising scheme for you to try. Whether you choose Google Adsense, Facebook, Reddit, or some other medium, you need to know first that you are getting something out of your investment. Read up on the nitty-gritty of your chosen method before you jump into it so you will not be spending for those precious clicks without getting anything in return.