How do I return money to a customer who I overbilled by $150. I invoiced the client and then charged their credit card

Let's record this transaction to your account, @sandra-verbeek-p.

You can write a check in QuickBooks Online (QBO) to record the refund. Then, link that check to the customer's overpayment.

Here's how to write a check and use Accounts Receivable as the account for the overpayment:

1. Go to the + New button and choose Check. 
2. Set the customer that you’ve overbilled in the Payee ▼ drop-down.
3. In the Payment account ▼ drop-down, select the bank account where you’ve deposited the overpayment.
4. On the first line of the Category column, select an Accounts Receivable.
5. Enter $150 in the Amount field.
6. Add all the necessary information.
7. Click on Save and close once you’re done. 

You can now link the refund to the customer's overpayment by receiving the payment:

1. Go back to the + New button and click on Receive payment.
2. Choose the same customer you used for the check.
3. In the Outstanding Transaction section, select the checkbox for the Check you created.
4. Fill out all the necessary information about this refund.
5. Click on Save and close once you’re done.

Check this article for more information on how to handle customer overpayment in QBO: Handle a customer credit or overpayment in QuickBooks Online.

In case you have QuickBooks payments and you process the payment there. You can follow the steps provided in this article on how process a refund: Void or refund customer payments in QuickBooks Online.

Let me know if there's anything that you need with your transactions. I'm always here to assist. Have a wonderful day!