Advanced shipping to handle

Hi good morning everyone and happy new year to all the community
I’m having a really complex shipping matter to handle in Woocommerce.

I’m building a spare parts website for a client and get stuck in the shipping I don’t know how
to handle it. So I will explain the matter and hopes someone can help me.

All the client needs for the shipping :
1) – For the international shipping he wants to use Fedex and Chronopost (for fedex he wants
to be able to show the live rates for clients in the checkout).
2) – For the national he want to use Colissimo and Chronopost (Because we are in
France and they have better prices).
3) – And obviously a platform that can connect to woocommerce and import all orders,
so that when we will recieve an order the guys in the store doesn’t have to access
the wordpress dashboard but go to an external platform to be able to see all the orders
and be able to print the labels to begin shipping products.

I’ve tested a plugins named Flexible Shipping For FedEx, connected it with the api
from the client’s fedex account, And chooses weights based shipping to calculate shipping
rates and it works but the rates are not accurates.
For fedex api to reply with a live rate they want to know the weight and the Packaging
dimensions L×W×H. So I think the pluging is taking the weights and chooses between fedex’s standard
packaging.
The only problem is that spare parts have really different volume weight ratio : for
example they will be air filters that have a big volume but less weight and other spares
that have more weight than volume.
And let’s consider a client orders (1 air filter, 2 pistons, and 8 O-rings) I can do the sums
of product’s weight to get the final weight but how can I calculate the final packaging
dimensions to get a more accurates live rates from fedex API ??

Maybe it’s not the right way to think about the problem, hopes someone can rederect me
to the solution.