First order help/suggestions

Hi, I gently ask suggestions about my first order shipping (to Italy). About an order of 20 music CDs is better choose a volumetric weight fee or an estimated weight fee? I don't understand why when I manually calculate the total weight of my order (sum of single items weight shown on my storage page) it's 1756gr. but when I select them on the same page it shows 2410gr weight estimated, more than 600gr of difference... Thanks for attentions, I wish happy new year!

Friend, system will add the packing materials and the weight of the box