I overpaid a vendor's invoice. How do I record the actual overpayment so that the credit balance is available to apply to subsequent bills from this vendor?

It's great you've come to this forum, nacs2013. I can help you record the actual overpayments from a vendor's invoice and add these credit balances to future bills.

First, let's create a vendor credit:

1. Go to the + New icon and click Vendor credit.
2. In the Vendor ▼ dropdown, select your vendor.
3. Depending on how you record purchases with this vendor, enter the Category details or Item details. Usually, this is the category, product, or service you’re getting credit for.
4. Once done, select Save and Close.

Then, apply credits to the subsequent bills from your vendor:

1. Go to the + New icon and click on Pay Bills.
2. Select a bill for your vendor from the list. You'll see the available credit with this vendor in the Credit Applied field.
3. Complete the rest of the fields like you normally do.

Feel free to browse this article for more information about vendor credits: Enter vendor credits and refunds in QuickBooks Online.

Moreover, I've added this reference to guide you in handling credits if you want to give credit memos to your customers:  Create and apply credit memos or delayed credits in QuickBooks Online.

I'd be delighted to assist you further if there's anything else you need additional assistance with. Feel free to reply on this thread, and I'll be around whenever you need help. Have a good one!