If I mistakenly transacted an ACH, can I just delete it and that won't charge the client's account? Please help

Let me share more information and steps to void the sales receipt, Finance.

To ensure your customer won't charged twice, we'll have to void the sales receipt and delete it before the payment transaction is batch at 3:00 PM PT. You can follow these steps:

1. Go to Sales, and then click All Sales.
2. Find the sales receipt you want to void.
3. In the Action column drop-down arrow, select Void and then choose Yes.
4. Once the sales receipt has zeroed out, click the drop-down arrow again and hit Delete.

However, if the transaction is already batch, we'll need to process a refund to your customer. To refund that payment, here's how:

1. In the left panel, click +New, and then select Refund receipt.
2. Select the customer you want to issue the refund in the Customer dropdown.
3. Fill out the with the same info on the original sale
4. On the Refund from dropdown, choose the account you want to issue the refund.
5. Hit Save and Close.

For more information, visit this article: Void or refund customer payments in QuickBooks Online.

Moreover, to customize your estimates and invoices, you can check this article: Customize invoices, estimates, and sales receipts in QuickBooks Online.

Let me know if you still have questions about your sales receipt. Have a great day ahead, and take care.