Hi there, usermorita.

Generally, pledges will appear in QuickBooks Online when a user like you has selected their organization as a non-profit. With this, removing the word "Pledge" from the invoice email subject line is currently unavailable. However, you can change the name Pledge to Invoice from the Email Subject template. To do this, you'll need to go to the Custom Form Style.

Here's how:

1. Select the Gear icon and click Custom Form Styles.
2. Click New style in the top right-hand corner and choose INVOICE. 
3. Choose the Emails tab.
4. Change the Pledge name to Invoice under the Subject box.
5. Hit Done.

Once done, you'll just need to manually select that template when printing or emailing the transaction. This is to make sure that when sending an invoice email, the word "Pledge" will not show.

For additional insight into customization, such as the different options available, you can click this article: How to Customize Invoices, Estimates, and Sales Receipts.

Please refer to this article to see steps on how you can receive and categorize invoice payments in QuickBooks Online: Record invoice payments in QuickBooks Online. This contains information on how to record single and partial payments.

If you need further assistance while working in QuickBooks, post a comment below. I’m here to help. Enjoy the rest of the day.