How do I to configure QBonline so when a customer uses a credit card, the calculated fee or at least a fixed convenience fee is added to their invoice, but not w/ACH?

To achieve your goal, you can add a convenience fee as an item line when sending an invoice, consulting2. I can walk you through the steps.
 
Please know that there's no direct setting to enable a processing fee to be directly applied when generating invoices. Alternatively, we can create a service item and manually include it in your invoices.
 
To do so, let's create a service fee item first:

1. Go to the Gear icon, then Product and services.
2. Click New and select Service.
3. Enter a name for this item. Example: Convenience Fee.
   
4. You can also disable or enable tax for this fee from the Sales tax section.
5. Input the fixed Sales price/rate or leave it blank.
6. Fill out the rest of the form appropriately, and once done, hit Save and close.
   

After that, you can then add the fee as an item line when creating an invoice to your customer:

1. Go to the + New menu, then Invoice.
2. Add your Customer, Invoice date, and Due date.
3. Input the invoice's Product/service on the first item line and the rest of its details.
4. Select the Convenience Fee item on the second line and enter its amount.
5. Double-check everything, and once ready, hit Save and close or Review and send.
   

    

For more details about this process, check out this link: Manually add service fees to invoices in QuickBooks Online.

Finally, you may also apply retail delivery charges and late fees to overdue invoices on the platform. Once you need to, you may review these articles as your reference:

• Track retail delivery fees in QuickBooks Online.
• Apply late fees to overdue invoices automatically in QuickBooks Online

Please know that you can always drop by again to this thread should you have more questions about service fee application to invoices. Our team is always committed to helping you promptly.

@consulting2 

If you are B2B, you can use a 3rd party processor to integrate with QB and accept any payment method (including ACH) for free.