We currently have this policy but have to manually insert a shipping line item on low value orders. Having free shipping on all BTB orders over $300 for example would likely increase the total order amount for some customers/orders.
We are currently building a zonal shipping service which will allow you to do exactly that: specify sales order price brackets and define shipping fees for each of them. This will be available both in TradeGecko B2B and on manual sales orders. Please stay tuned for the announcements!