There are many different companies which offer direct mail services. Stannp.com for example is a web to print mail house which enables companies to send out their direct mail all from a simple web site. You can just upload your template creative and a set of data. You then review and approve the mail piece as a PDF doc and then schedule pay and launch the campaign. The letter or postcard will hit the door mat in the UK 2 days later or 5 days for international campaigns. The cost for a thousand letters or A5 postcards is 41p. This includes printing and delivery.
You might want to send an email campaign and there are many companies which will either send a campaign to your own data (like sendgrid or mailchimp) or they will send a campaign for you to their own data and attempt to drive you new customers like web clubs, offerX and Submission technologies. To send an email to your own data you are looking at a cost of about 50p per thousand emails dispatched. If you are looking for an external company to drive customers to you then they will charge about £5 per thousand emails depending on the recipient profile.
You could be looking at sending a text message and there are many suppliers who will either send the campaign for you (so they are a gateway) or other companies who will send your campaign to their data and drive you customers. The cost of this is about 1.8p per text sent and the data will cost about £5 per thousand records.
There are also creative agencies which can design you mail piece and they will charge on an hourly rate. There are some new online tools like Canva.com which