Service in assembly

We are a manufacturing company and use inventory parts and assembly builds. Our specialty is the infusion of our products.

I'd like to track the infusion cost in the assembly item. I'd like to know how to best do this. It's not Labor. It's just the service we provide via our technology.

What accounts do I need to use?

Currently the Expense is COGS and the Income Account sales. However, it throws my inventory summary off. It never matches the balance sheets inventory asset account. After doing some digging, it seems to be this particular service item.

Any suggestion?