how to bill a customer for reimbursement of expense?

When billing customers for purchases you've made on their behalf, there are several factors to take into account, Alida-Nanjee.
 
To begin, please note that generating an invoice for your customers in QuickBooks Online (QBO) will impact both your income and accounts receivable. To ensure proper and accurate recording, it is recommended that you consult with an accountant for guidance.
 
In the meantime, you can create a journal entry using the accounts receivable to ensure your invoices are recorded as receivables rather than income for your business.
 
To learn more about how to record business expenses paid with personal funds in QBO, please refer to this article: Pay for business expenses with personal funds.
 
Let us know if you need more information about invoicing your customer in QBO. We're always here to help.