If you sell a lot of items online then you need to try and keep your postage costs to a minimum. This is true if you also sell through sites such as Ebay where profit may be miniscule. You need to try and pack your parcel as small and as light as possible as postage is worked out on dimensions and weight of the package. You also need to ensure though that the parcel is packaged safely and securely as if it gets damaged in transit and you have not packaged it sufficiently then you will be liable to replace or refund the item.
IF sending something in a box such as medicine, you may be able to flat pack the items to reduce the postage costs. You should try and pass part of this reduced cost on to the customer. Many people are more tempted to buy from a business that offers free postage, especially if the postage is free on smaller / cheaper items as it can be very off putting if you want to buy something that is £5 but the postage is then £4. Often increasing the price of the product to cover the postage, sells better than charging a postage fee.