Workflow for Vendor purchase orders for inventory items that we supply parts and they build

We manufacture clothing and for every finished product we use built assemblies as each of them use inventory parts. We supply all the parts to a Vendor (contract sewing company) to be built and they charge us the labor for building the assembly. Their labor is currently in our BOM for each inventory assembly.

Currently our workflow is

1) Create a purchase order for each product/color/size we're ordering from the Vendor at the labor rate that they're charging us. So we can submit the order to them

2) When it's completed, we create the bill and pay them. Then we delete the PO and go into build assemblies and build the product. That way it pulls out the COGs for every finished good and the inventory parts used so we can track what's left in parts. Also, the labor isn't charged twice for the product.

Is there a better workflow or way to do this that wouldn't require us to run build assemblies and to receive the PO instead as inventory? If we pulled out the labor from our BOM and charged it through the PO would that still appear in our COGs for the product? The build assemblies are time consuming and prone to data entry errors since every SKU has to be entered one at a time.

Also, by deleting the PO there doesn't seem a clear way to calculate the landed cost for shipping the finished goods.

I've looked through posts and online and the closest question to this, it did not seem like there was an easier path.