The terms Billable and Non-Billable expenses are normally associated with say a consultancy company where they bill consultants out to their clients. Therefore Billable expenses will be paid by the client whereas Non-billable will be paid by your employer. What's included in both is subject to the contract your company has with their client.
For material/Goods Purchases, the Po is required. In the Po, QTY, Product Name, Where is delivery price, Terms and Conditions. In the po there is no matter of final amount. It is just the approximate amount.