The comments come as the Royal Mail and Communication Workers Union (CWU) continue with talks aimed at averting the national strikes planned later this week.
The advertiser body says if strike action is not avoided companies could move their business away permanently and turn from direct mail to email.
David Ellison, ISBA’s marketing services manager, says the strikes have already “undermined” the relationship its direct mail members have with the Royal Mail with alternative delivery services “becoming increasingly appealing”.
At time of writing, Royal Mail and CWU were locked in a second day of talks ahead of the announced action on 22 and 23 October.
News of the discussions came as it was revealed Royal Mail plans to hire 30,000 temporary staff to clear the backlog caused by the strike and deal with the busy Christmas period.
The planned strike has prompted retailers including John Lewis, Amazon, eBay and Argos to discuss using alternative delivery services over fears that the strike will seriously disrupt business in the run-up to Christmas.
Research carried out by the Centre for Economics and Business Research for Kelkoo has estimated that a nationwide strike could cost UK businesses £1.5bn in lost revenues, with the cost to the retail sector about £270m.