Here is a simpler explanation: If your client is reimbursing you for your expenses, you record the expenses as billable expenses when they occur. With Quickbooks Online, here is how you do that: (Quickbooks Online Plus/Advanced required): First, you make sure billable expenses are turned on in your settings (under Your Company, Account and Settings, Expenses, then turn on the settings to track expenses and items by customer. 

Next, When entering the expense transaction, by each line of the expense, you can check the billable box, choose the customer name that it is billable to, and calculate the amount of markup you are charging the customer on the expense.  

Once you have recorded billable expenses in this way, when you invoice your customer that has an unbilled billable expense, you can add the billable expense to the invoice.  Sadly, at this time Quickbooks does not let you add these to sales receipts.