Packing And Mailing...
is the core of your business. As simple as this may seem, it's what forms the nucleus of your operation and brings in the majority of your customers.
Just like the post office, you supply stamps; but in addition, you provide envelopes, cartons, packaging materials, and more. You offer custom/specialty packaging services, enabling the customer to simply bring in the item to be shipped.
You do the work for them, and you make money.