UN 3108 is Organic peroxide Type E, solid (……) and according to DGR 4.2, we use PI 570, and the maximum allowed weight for a passenger carrying aircraft is 10kg. So, 6.4kg is within limits. PI 570 permits 4G. We need to follow SP A802 which says the packaging performance must satisfy PG II. This means Y is okay.
The 6.2 is the reason of rejection. UN 3108 itself weighs 6.4kg. This cannot go into a packaging that was tested for maximum 6.2kg. The weight is insufficient. The weight of the inner packagings, cushioning material, absorbent material, and the weight of the outer packaging are not considered. Taking all such into consideration, you will need to bring a 4G tested for at least 15kg (Y15) to stay on the safe side.