I second the last comment. I’ve also been told Shopify are rolling this out natively but waiting on clarity on date. They will need to roll it out on time since all merchants need it.
What we did as a temp solution (and this would be a Plus only solution) was use the ajax api to add the line item which takes care of all orders except accelerated checkout. We used the Flow app to manage orders without it by editing orders after the event.
I’d wait for this to be done natively and not use an app tbh.
In my experience (10 years plus of Shopify dev) shipping apps can be clunky/slow and sometimes don’t even load rates at all.