Thank you for posting here in the Community, @JMackeyCPA.

Let me lay down some information about applying discounts to your customer's invoice in QuickBooks Online (QBO).

You're right that there isn't an option to automatically apply discounts based on a set number of days. The ability to automatically apply the discounts is available in QBO, though. There is only a slight difference between the choices you prefer.

If you want a discount to be applied automatically, you’ll want to utilize the price rule feature in your company file. This way, you can set a fixed amount, custom price per item or percentage to be discounted from your customer’s invoice. You can also offer them for a set period to selected customers if you prefer.

Let’s turn on the feature by following the steps below:

1. Go to the Gear icon and select Account and Settings.
2. Select the Sales tab and click anywhere within the Products and services to Edit.
3. Toggle on the Price rule to enable this. Then, select Save and then Done.

To create a price rule, here’s how:

1. Go to the Gear icon and select All Lists.
2. Select Price Rules, and click the Create a rule button.
3. Enter your rule name and fill out the fields accordingly.
4. Once done, select Apply. Then, Save and close.

I also added this resource for more information: Set price rules in QuickBooks Online

We can also send feedback to our developers on adding the option to automatically apply discounts depending on a certain number of days. Simply go to the Gear icon, then select Feedback.

Furthermore, browse this article to be more knowledgeable when receiving and recording payments in QuickBooks Online.

Let me know if you have more concerns about applying discounts. The Community is always here to assist you. Have a good one!