The shipping costs you paid cover the shipping company fee, insurance, and any packing and handling.
So if the amount you paid is higher than the amount on the shipping invoice, the extra cost might be to insure the shipping and to cover things like boxes, tape, bubble wrap or packing foam. For very fragile items, sellers might even need to custom-make the packaging to ensure safe shipping.
If you have any questions about these extra costs, reach out to the seller to find out more.
Keep in mind, if you live in a different country to the seller, you might also need to pay customs fees.