I appreciate you for coming back to the thread and clarifying your concern, 123134. Please know that there's a way to have the client's name show up in the subject line automatically when sending an invoice in QuickBooks Online. I've got steps to ensure you can achieve this process.

To start with, you can open your created custom template and add the client's name in the subject line field. Then, make sure not to remove the word Invoice from the field to save the changes.

Here's how:

1. Click the Gear icon and choose Custom form styles.
2. Locate the specific template and click the Edit link.
3. Choose the Emails button. Then, go to the Subject box and enter the client's name.
4. Click Done.

On the other hand, you also have the option to perform the process in the Messages section. You can follow the steps below.

1. Click the Gear icon and select Account and settings.
2. Choose the Sales tab.
3. Go to the Messages section and click the pencil icon. Then, enter the client's name in the Email subject line field.
4. Click Save and Done.

Furthermore, please note that the updated info is automatically applied to all invoices. In this case, you need to manually edit the template and modify the name whenever you need to send the transaction to another person. 

In case you need steps on how you can manage the status of your sales transactions like open and paid invoices in QBO, you can read this article: View sales transactions in QuickBooks Online.

If you still have questions about customizing invoices and other related concerns, you can leave a comment below. I’ll be glad to answer them for you,123134. Have a good one.