Is there a way I can collect a retainer fee and have it on the next invoice without manually entering the retainer?

Not sure if it will help, but I found a thread that might help. If not, you can always use the Search bar at the top of any page, here in the community, to look for a better answer. 

https://quickbooks.intuit.com/learn-support/en-us/account-management/correct-way-to-enter-a-retainer...

Hello there, @Kharpel. 

I understand how convenient it is for you to automatically apply retainer fees in your transactions. At this time, only credit memos can be applied automatically to the next invoices. To enable the auto-apply feature in credit memos, here's how:

1. Go to the Gear icon ⚙ and click Account and settings.
2. Click the Advanced tab.
3.  In the Automation section, select the pencil icon ✎.
4. Turn on Automatically apply credits and select Save.
5. Once finished, click Done.

For the time being, the option to automatically apply retainer fees is unavailable in QuickBooks Online. However, you may always submit a recommendation to our product engineers to address your concern. To submit feedback, please follow these steps:

1. In your QuickBooks Online account, go to the Gear icon and select Feedback.
2. Type your suggestion in the box and click Next.

You may also be interested in creating templates for recurring transactions. Here's the article to see the complete steps: Create recurring transactions in QuickBooks Online.

I appreciate you for bringing this issue to our attention. For further concerns, please don't hesitate to leave a reply. Keep safe!