How do I add costs to a service item that I sell?

The ability to include cost in a service is available in QuickBooks Online Plus and Advanced, AJC13. If you're using one of these versions, let's edit the item you sell to your customers.

Before proceeding, please know that the Purchase cost is for a service bought from a vendor. Since you sell it to your customers, I'd like to verify if you're referring to the Price/rate field. If so, follow the steps below to modify the service item:

1. Go to the Sales menu on the left panel.
2. Navigate to the Products and services tab to select the service item.
3. Tick the I sell this product/service to my customers checkbox.
4. Enter the amount in the Price/rate field.
5. Click Save or Save and close.

However, adding a cost for a service item in Simple or Essentials versions is unavailable. You'll want to consider upgrading your subscription by following this article: Upgrade or downgrade your QuickBooks Online subscription.

For more insights on tracking product and service items you buy and sell, feel free to review this material: Add product and service items to QuickBooks Online.

Once done, create and send invoices to charge your customers for the services you market. 

Do you need assistance with accounting sales? Talk to our QuickBooks Live Expert Assisted now to receive appropriate guidance on monitoring your business finances.

Keep me updated if you have further questions about managing service items or handling invoices. I'll be happy to help you again.

Your costs should be somewhere in your books, but there's probably no reasonable way to associate them with the item in QuickBooks Online. Naturally there is with QuickBooks Desktop, through the time tracking and payroll features, but I don't think that's possible in QuickBooks Online.