The best retail campaign is usually the smallest one
Retailers instinctively measure a campaign by how many people it reached, which is exactly backwards. The sends that consistently produce revenue are the small ones: a few hundred people who are due to repurchase, a few dozen regulars who have gone quiet, the customers who bought in a category where something genuinely new has arrived. They perform because the message is relevant, and relevance is a function of knowing what somebody bought.
The broadcast has its place, mostly around festivals and seasonal events where everybody expects to hear from shops anyway. But a retailer whose entire programme is broadcasts will find the response declining every year, and will usually respond by sending more, which accelerates the decline.