How to realize the process for having "bottle-washing" service outside

Dear Gurus,

Here in our company, we would like to realize the below process in SAP.

1. We create a Purchase Order(PO) to buy some bottles from the supplyer. (PO Creation)

2. The supplyer delivers the bottles as per the PO and we receive them. (Post Goods Receipt)

3. These bottles need to be washed up once we receive them. So we store them as blocked stocks.

4. In order for the bottles to be washed, we will create another PO to have the bottles washed by the service vendor. (PO Creation)

5. At the same timing, we pick up those bottles and send the bottles to the service vendor. (Post Goods Issue)

6. The service vendor washes all the bottles and send them back to our company and we receive them. (Post Goods Receipt)

7. Once we receive them, we will keep them as unrestricted stocks.

Number 1 to 3 are nothing but normal process.

But from 4 to 6, I would like to know how to do so...

I am assuming the PO to service vendor should be sub-contract one, but in this case the BOM should be recurrsive, because what we send to the service vendor and what we receive are exactly the same bottles, aren't they?

And since this is just for having outsourced service, we only want to pay for the service itself, not for the price of the bottles.

Hence I am wondering how to avoid having the bottle price in the condition view and also how to include service price for washing and shipping cost. Do we have to maintein service master for it?

Please advise me to have those process in our SAP system, hopefully step by step.

Thank you very much for your support in advance!