I've got your back in handling overpayments for both your customers and vendors in QuickBooks Online (QBO), @htao2013.

There are different ways in handling a customer overpayment in QBO. The example provided by my colleague above applies to the previous customer's scenario. In your case, you can either use the credit toward an invoice, refund the customer, or enter the overpayment as a tip. This way, you're able to keep your customers and vendors separate.

Once you opt to use the credit toward an invoice, you'll have the option to apply it to an existing transaction or a new one. To apply the credit to an invoice you already created, here's how:

1. Go to the + New button.
2. Select Receive payment.
3. Choose the customer, then the credit, and the invoice.
4. Select Save and New or Save and Close.

I've attached a screenshot below that shows the last two steps.

For the step-by-step guide in applying a credit to a new invoice, you can refer to this article: Handle a customer credit or overpayment in QuickBooks Online. It also includes details about setting up customer credits to apply to future payments by default.

On your vendors' side, how you enter the credit depends on how you record your purchases. In case you want to be sure, you can also refer to your accountant to further guide you in keeping your financial data accurate. Then, refer to this article to choose the appropriate steps that apply to you: Enter a credit from a vendor.

Also, I'm adding this article to further guide you in managing both your customer and vendor transactions using QBO: QuickBooks Help Articles. It also includes video tutorials to know your way around the product.

Let me know in the comments below if you have other concerns about handling overpayment transactions in QBO. I'll be glad to help. Take care always.