How can I accept payments from overseas via ACH with little to no fees (only flat fees if any)? Specifically from UK/South Africa

You can accept payments using the following payment methods available in JotForm:

You may want to try Paypal as it is most commonly used by our users. To setup a payment form, you can check this guide for more information: Setting Up Your First Order Form. Regarding the fees, I think most of the payment gateways are asking service fees. You may want to check their sites for more information about their fees. For Paypal, you can check this site: https://www.paypal.com/ph/webapps/mpp/merchant.

That didn't at all answer my question.

Unfortunately, the mentioned Payment fields show in the screenshot above are the only available payment methods we have. As per checking, Paypal and Stripe accepts ACH payments. However, the payment fields we have only accepts credit cards. If you want, you can try to use the Purchase Order field so you can setup the product for your form. Then manually email the submitter for the payment method.