In sales, getting an excuse for not delivering the materials on time to customers - What will be the most suitable reason that can convince the customer?

Apologize sincerely and focus on what you can do rather than focusing on what you cannot do. 

Under testing for Quality control and Quality assurance..

Always keep your customer mindset that you and your company are taking quality a more serious thing in policy.

Good evening! An apology is inevitable.No matter whatever reason we give, the customer will always have a scar in his mind about us. So weneed to do something about it. Now as you sow so shall you reap, since the delay happened from our end, we can check whether the customer is effected due to it and if there is anything which we could do,then we should do it as an exception.

For eg, the customer orders a TV, it was to be delivered in a week, unfortunately it was delivered after 10 days! The customer may have moved to a different location.In this scenario, we should deliver it to his new place without any extra charges!

If there is nothing which we can do,then atleast a box of chocolates or a bouquet with an apology message could help.

First of all to apologize the customer for the delay. 

And secondly, i would be telling that the delay has made been because we do not want to compromise on the quality and service and that is why we are working till perfection. Be rest assured.