Thanks for taking the time to reach out to the QuickBooks Community. I hope you're having a great day so far, @saloninsandiego. Currently, customizing the remittance advice template isn't possible.
You can, however, manually edit the Subject and Message template before sending it to your vendors. 
Alternatively, you may want to explore third-party applications that fulfill this functionality in QuickBooks. You can check the Apps section, or by visiting this page: QuickBooks Apps Store.
In the meantime, I suggest submitting direct feedback to our Product Development Team to evaluate this function. If they determine that it could enhance our users' experience with sending remittances, then they can add this to our future product updates.
- Sign in to your QBO account and go to the Settings.
- Select Feedback below the Profile column.
- Enter your product suggestions. Then, click on Next to submit it.
You might pull up a Transaction List by Vendor report to generate a page that shows vendor payments, bills, and status. Check out this article to learn how: Run a report in QuickBooks Online.
Click the Reply button below if you have additional questions about sending remittances or customizing vendor transactions. We'll be willing to lend a hand. Have a good day.