Yes, you can set the ACH as default payment method if you have QuickBooks payments subscription, @Art013. I'm glad to outline the complete steps to achieve your tasks. 

You need to edit the invoice template and choose Bank Transfer as the accepted form of payment. This way, your customers' primary payment method is ACH.

 Here's How:

1. Go to the Gear icon at the top and select Customer Form styles.
2. Find the template, then click Edit under Action.
3. Go to the Payments tab and ensure only Bank transfer is checked.
4. Click Done. 

Also, you have the option to set that template as your default. This way, we can ensure that your customer can pay you via ACH. Just click small drop-down arrow beside Edit and  select Make default. See the sample screenshot below for your reference:

For more details, see this article: Customize invoices, estimates, and sales receipts in QuickBooks Online,

Check out these articles below to get answers to the commonly asked questions about ACH payments in QuickBooks:

• Accept ACH payments and eChecks
• Standard Payment Fees & Rates

If you have more questions about QuickBooks, feel free to come back. The Community is always around to help in any way that we can.