Hi I am using Australia post and sending around 30 to 40 parcels per week , can anyone suggest any cheap shipping service, also I can see some sellers selling same item which I am selling at very competitive price as supplier is same , I just wandered how they sell somthing for $11 with postage item weight is 500gram but it cost $4 how they manage to sell it for $11 after paying postage and other fees

thanks