Hello there, @SmokinB. I appreciate you for joining here.

I'll be providing details about adding items when utilizing the Add Time/Cost option on the invoice, and how QuickBooks works on hiding the mark-ups on invoices.

In QuickBooks Desktop, the items will only generate if you have a timesheet or transaction billable to the customer. In this case, you can create a bill and make it billable to your customer. Here's how:

1. From the Vendors menu, select Enter bills.
2. Choose the Vendor.
3. Enter the Date.
4. Fill in the appropriate details in the following fields:
   - Account: Make sure to choose the correct account.
   - Amount: Enter the total amount of the bill.
   - Memo: You can leave this field blank if necessary.
   - Customer:Job: Choose the customer billable for this item. Choosing the customer will automatically mark this item as Billable.
5. Click Save & Close.
   

Now, we just need to create the invoice for the customer with the marked up amount. Here's how:

1. Click the Customers menu, then select Create Invoices.
2. Enter the customer that you entered on the bill.
3. Click the Add Time/Costs.
4. In the Items tab, select the item. Then, OK.

QuickBooks gives us the option to hide the mark up added on the invoice. However, the hide option only applies when printing them. With that being said, your client will be able to see it. 

If you consider doing this, make sure to uncheck the  Print selected time and costs as invoice item box at the bottom. See the sample screenshot below for reference.

Also, you can check this article for more insights and tips: Tracking job costs in QuickBooks Desktop.

If you need anything else with QuickBooks Desktop, please let me know by commenting below. I’ll be around to help. Take care and stay safe always.