Hello , can any body tell me how to linkthe shipping invoice to the main invoice of products to get the real cost of goods

Let me elaborate on some details about how you can get the real cost of goods for your products. I'm here to guide you on how.

If you're referring to the shipping fee that you want to charge to your customer, you can create a service item on your product and services page. To do so, you can follow the steps below:

1. Go to the Gear icon, and select the Product and service page.
2. Select New.
3. Select Service.
4. Enter a Name for the service.
5. Enter the necessary information such as the Category, Sales price/rate, and income account.
6. Click Save and close.

Once done, you can use this service item when you make an invoice for your customer. 

However, if you're allocating the shipping cost to a specific product, this isn't possible at this time. You'll want to search for a third-party application online that you can use to integrate with your QuickBooks Online (QBO) account. 

For more information on how to track and how much you make and spend on each product and service, check out this article for more details: Add Product And Service Items To QuickBooks Online.

In addition, learn from this article how you can add the product and service you're selling to an invoice: Create Invoices In QuickBooks Online. 

Keep me posted if you're referring to something else. I'll always be right here to help you all the time.