Which Shanghai Postoffice accepts international Parcel of 20 or 30 kg? From Shanghai to Switzerland

Parcel of both 20kg and 30kg can be delivered and the cheapest way is the surface mail, which cost around CNY550 for the 20kg package and CNY780 for a 30kg parcel. You can certainly buy the box at the post office.
CN Post provides Pick Up service and it won't charge extra fee.