Indeed I do not think we must complicate the carrier price computation with a MatchMixin or anything else. This is for two reasons:
- We must be able to compute price also with only the shipment data
- The base prices of shipping are based on other information than the sale amount (weight or value)
So for me the free shipping should be managed as a promotion based on the initial amount (new feature).
Of course such promotion would only work if the shipment cost method is based on order (and it does not really make sense when based on shipment).