If it's the slightest under 8/8 I would always say 'a tad under under 8/8' and give the exact measurement. In your case 63/64. But I have to agree with Nelson. The buyer didn't like it and just used the excuse of the width to return it.
Don't forget that not everyone has a caliper so it might be 63/64 for you but for the buyer who just uses a ruler at eyesight it might look 15/16.