A customer wants to update their fancy accounting system with ACH payments for me. Where can I find the banking information they need to pay me using ACH?

Do you have QB Payment account or using another payment processor?

I have a QB payment account.

Hello, Feike.

Thanks for reaching out to the Community for support. I'd like to help you out with your ACH payments. 
 

If you have QuickBooks payments you'll need to ask for the bank information of your customer. Once done you can now process the payments.
 

Here's how:

1. Click + New.
2. Hit Sales receipt or Make a sale.
3. Choose the customer from the Customer dropdown.
4. Add the products and services you're selling.
5. Fill out the form.
6. In the Payment method dropdown, click the payment method.
7. When you're done, tap Save and close.
   
   

I've added this article for more details with find matching transactions: Add and match downloaded banking transactions.

You can always post again here in the Community, should you have any other questions concerning payments. Helping you is my priority. Have a good one!