We are a trading company .how do I assign products and services to supplier and customer to see profit n loss by each shipment alone?

It's nice to have you here, maroof-mansoor-g.

QuickBooks Online tracks how much you make and spend on each product or service. You can also enter them as items you sell, so you can add them to sales forms. While there isn't a Profit and Loss report specific for products and services, you can compare reports to get the information you need.

The Profit - Sales by Product/Service Detail

1. Go to the Reports tab.
2. Type Sales in the search bar and select Sales by Product/Service Detail from the pane below.
3. Set the Report period to the date range you need to know the profit for and click Run report.

The Loss - Purchases by Product/Services Detail

1. Go to the Reports tab.
2. Type Purchases in the search bar and select Purchases by Product/Services Detail from the pane below.
3. Set the Report period to the date range you need to know the loss for and click Run report.

For more details in setting up product and service items, please visit this article: Add Services and Products.

You can also customise reports in QuickBooks. Then, memorise them to save the current customised settings. 

I'll be glad to help you again if you have more questions about running reports. Just add the details in the comment section.