Hi there, @Luluwawa.

I'm here to help in importing your invoices with custom fields.

In QuickBooks Online (QBO), there are required invoice fields when importing that you need to use. Thus, your custom fields should match what's in QuickBooks for you to import successfully. 

For guidance, you can download this sample spreadsheet to map each column headings.

Once you're done reviewing your spreadsheet and is ready for upload, here's what you'll need to do.

1. Click the Gear icon at the top.
2. Select Import Data.
3. Choose Invoices.
4. (Optional) Under the Browse button, you can select the checkboxes to add new customers, products, and services that don't already exist in QuickBooks.
5. Click Browse and then find and select the spreadsheet of your invoices. 
6. Click Next.
7. Map your fields, then click Next.
8. Click OK, then Done.

For more details about this process, see this article: Import multiple invoices at once.

To learn more about importing data in QBO, here's a link that you may find helpful: Common questions about importing data to QuickBooks Online.

Let me know if you have any other concerns or questions. I’ll be right here to answer them for you. Have a great rest of the day.