How to record purchase discounts so that it reflects on the COGS?

You'll want to add a purchase discount item and assign it to the COGS account, @Canahmad.

Let's go to the Products and Services page to add new products or service items so you can easily keep track of your transactions in QuickBooks Online (QBO).

You can enter products and services as items to quickly add them to sales forms. This way, you can have more detailed financial reports and complete transactions faster. To add a new purchase discounts item and assign it to the COGS account, here's how:

1. Go to Products and services.
2. Click New.
3. Select Non-inventory or Service.
4. Add a name, for example, Purchase discounts. If you track SKUs, enter an SKU for the product.
5. From the Category dropdown, select the category that best describes your product or service. 
6. Select the I sell this product/service to my customers checkbox. If you don't sell the item, you can leave it unchecked.
7. Enter a description. This is what your customers will see on their sales form.
8. Enter an amount in the Sales price/rate field. 
9. Select the Income account dropdown and the account you want to use (COGS). If you haven't set up the account yet, select the + Add new option to create.
10. Click Save and close.

Once you're done, add the purchase discounts item to your invoices, sales receipts, expenses, and other sales forms.

You can read this article for the complete guide: Add product and service items to QuickBooks Online.

Also, you may want to check out this article as your reference to guide you in getting insights on the things your buy and sell and the status of your inventory in QBO: Use reports to see your sales and inventory status.

Let me know if you have other concerns about managing sales and expense items in QBO. I'll gladly help. Take care, and have a great day, @Canahmad.