Shipping Outbound Weight Fee


I have problem i dont understand how to calculate the Charge Fee Shipping Outbound Wight for example if i sell product like Iphone Charger and the Weight of each product 0.30 Gm including each box unit and the size for each unit including each box unit under 18x14x8 But the Outbound Shipping Mater Box including 5,000 Pcs From China over 20 Lbs and over 18x14x8 Size so how Amazon Charge me Small Standart Size Large Standart Size or Over Size?
Thank All!