How do I create a job number, then allocate to a customer so I can then allocate bills for cost of sale?

Hi Anna, 

Thank you for coming to our community site for your query.

Based on what you are trying to achieve, I would suggest using our "Billable Expense" feature in QuickBooks Online. You can create a bill which then populates a bill number, you can have this as a job number and you can then create invoices from the billable expenses you raised. 

Below are the detailed instructions on how to enable billable expenses and the process on how to create them. 

https://quickbooks.intuit.com/learn-support/en-au/manage-suppliers-and-expenses/enter-billable-expenses/01/263254

If you need further assistance, please do not hesitate to get in touch with us. Cheers!