You need to be 100% certain of what you can reasonably provide before you offer to provide it! Once you have that determined and well thought-through, your best answer is a 100% satisfaction guarantee for the customer. For instance, my company sells imported baked goods from Europe. Someone can call us and say, 6 months down the line, that the pastries we sent (with a 3 week guarantee) are stale we'll still serve her.
With this in mind, it's vital that you have well-trained, responsible and fair-minded representatives on the job. Not every rep can be great, but they need to have senior reps there to do the 'dirty work' for them. I'm a supervisor with 10 years on the job, and that issue of stale pastries from 6 months ago was resolved by me, 1) knowing we have enough mark-up to deal with the loss if it equals continued patronage, 2) the confidence of my employer to trust my decision. We wound up with a satisfied customer and repeat (non-problematic) business.
I'm assuming this question is being asked by an employer, and if that is the case, my biggest suggestion is to put the effort into choosing the right reps and treating them with the respect they deserve. For reps, treat your customer like they are paying your rent, because in the end, they really are.
Above all else - you must have a robust process for getting customer feedback and applying it your business - without is your too often reacting to exceptions or jumping at shadows