Radio is a perfect complementary medium. It can help direct mail. Radio as a standalone medium can generate a 29% lift in search.
Consumers hear only one radio ad at a time, in contrast to the clutter of many pieces of mail delivered at once.
Radio generates new customers by branding an advertiser. Consumers who are familiar with a brand may be more receptive to direct mail pieces they find in their mail. Radio ads run concurrently with a direct mail campaign can educate consumers about the business and direct them to check their mailboxes for more detail, ordering, etc. Radio can amplify a campaign's impact. In fact, radio can drive awareness lift of a direct mail campaign by 78%.
Radio Ad Lab and other studies since then have established that listeners feel a personal connection to the radio stations they tune to, so hearing an ad on radio and finding a direct mail piece in the mailbox for the same business may alleviate the perception that they're receiving junk mail.
For more information on radio:
Statistics and Trends: http://www.rab.com/whyradio/