They were told that they would be compensated for the costs of delivery and other logistics, but not for the cost of printing the items.
This means door-drop firms could lose out, especially in the case of having to compensate clients who planned to send time-sensitive creative, which it would be impossible to deliver at a later date.
The CWU has proposed two 48-hour strikes for October 5-6 and October 8-9. Talks between the two sides have proved unable to generate a compromise.
Action is also being threatened by the Royal Mail managers' union Unite to demonstrate its concern about Royal Mail's plans to make its pension scheme less generous.
Ben Allan, managing director of Tilt, the company behind the Asrecommended series of financial guide door drops, said his company would have to cover the cost of printing 1m items.
Allan said: "I can understand this is increasingly frustrating for Royal Mail because the delivery of unaddressed mail is a lucrative and profitable operation and they will have been loath to make such a decision.
"But equally, the impact will be greater on the door-to-door industry, especially in the collaborative marketing sector, because we have to compensate the advertising costs of up to 15 clients who advertise in the Asrecommended publications.
"Moreover, and contrary to similar situations in the past, customers affected by the strikes are having to incur the printing costs. In terms of asrecommended this means covering the cost of up to 1m magazines that were due to be distributed throughout October."