Sources:

www.tmrdirect.com/6-reasons-your-direct-marketing-strategy-should-include-coupons

www.themailshark.com/resources/blog/send-direct-mail-coupons/

www.businessknowhow.com/marketing.couponing.htm

www.valpak.com/advertise/products-services/direct-mail-coupons

fitsmallbusiness.com/coupon-advertising/

articles.bplans.com/tips-for-using-postcards-as-direct-mail-coupons/