App to bulk edit item prices in shipments

My use-case

In shopify, I charge my customer 450€ for my product. This prices is inclusive of a shipping contribution, taxes, and all import duties — not itemised. The customer sees 450€ on the product page, sees that the price is inclusive of taxes, and see one "Free" shipping option. They pay 450€ as the shopify product price and nothing else.

When I ship this product to my customer, internationally and in DDP, my commercial invoice, upon which I will be charged import duties, should not list this item at the full 450€.

Instead, it should list it at 450 minus shipping contribution minus taxes. In my case, that is 380.

The rest of the money I charged will go towards paying shipping and duties.

As I understand it, I have to feed €380 to Easyship either by changing the order. or shipment.item.declared_customs_value for each item.

(Or perhaps I should be looking at Tax and Duty > item.customs_value? Or are those one and the same?)

As I understand it, I have no way in the UI to do that in bulk.

Explored solutions

Shopify does not allow me to bulk edit my Oders (eg via their Bulk editor or an automation in Shopify flow) either.
I see I could work without Synced Orders between Shopify and EasyShip. Instead, periodically I would:

Export my shopify orders to an Airtable
Via an Airtable automation, I update my items and total prices
I export my Airtable to csv (everything until this step can be automated)
I import the csv into Easyship as new shipments

But it also looks like this is doable via the Easyship API.

How should I be approaching this? It doesn't appear doable with something like Zapier.

I have budget for development.

Thank you for your advice in advance.