It's good to see you enhancing your accounting process by transitioning to an online platform, @e_hatfield.
Once you migrate from QuickBooks Desktop (QBDT) for Mac to QuickBooks Online (QBO), your billable expenses will automatically transfer over as billable expenses in the Online version. You can read this article to learn more about which features and data will transfer and which will not: Learn how features and data transfer from QuickBooks Desktop to QuickBooks Online.
After your migration, you'll have to enable the markup feature in QBO. This way, when you convert the billable expenses into an invoice, the markup will be automatically calculated.
Here's how to enable the markup feature:
1. Click the Gear icon in the upper right corner.

2. Select Account and settings.
3. In the Expenses tab, check the box for Markup with a default rate of from the Bills and expenses section and enter a percentage.
4. Click Save.

Once enabled, the markup will appear as a line item on your QuickBooks invoice, but it will be hidden when you send it to your customer or print it.
Please note that the billable expense feature is exclusive to QBO Plus and Advanced subscriptions. For more information about this, you can refer to this article: Enter billable expenses.
If you have any additional questions, please don't hesitate to reply below.