Hi Jasoh,
I just wanted to point out that this information is not correct. I am on a paid plan using Shopify Payments. The step-by-step instructions provided under customer rounding rules are not up to date, hence cannot be followed. I have just been informed by Shopify support that there is no possibility yet to add custom rounding rules. This is a feature that Shopify does not support at the moment.
If I was misinformed by customer support, please get in touch with me with instructions how to add custom rounding rules.
Looking forward to the feature release!
Thanks!
Silvio