Ability to pass ad-hoc shipping and tax amount using External Cart API

We are exploring External Cart API for our store to use Shopify as customer facing site and using Shopify cart/checkout processes for checkout experience. But we notices few limitations in Deco network External Cart API.
Is there any way we can pass shipping amount as is instead of shipping method. There is option to only send shipping method id?
Is there a way to pass Tax amount as well?
And any way to add ad-hoc order line item with amount like tip, donation etc which can be part of total order amount?