How to send parcels without losing money

We are a new start up clothing business.

We charge fixed delivery rates (standard next day etc).

However, how do companies get round sending larger parcels without losing money, e.g. if a customer orders 10 items and pays standard delivery charge? The bigger the parcel won’t more it’s going to cost to send and the company will lose money, especially if the items are returned.

For instance how do Asos, Marks and Spencer’s, Debenhams etc send large parcels without loosing money.

We would welcome any advice or assistance.

Thanks in advance