It used to be as simple as including an insert in the same envelope as your transaction documents. Things have now moved on to "onserts" or "TransPromo", where a promotional message is placed directly on the transaction document. This lets you include operational and promotional messaging on the same document. You save paper and put existing white space to good use.
This type of strategy will help you lower operational costs as it reduces the need to print on separate brochures. It could also save your customer resources, as they don't have to send as many email communications to existing customers.