
I would create an expense sheet and enter in the information related to the invoice there. You can create a custom field to track the sales tax to track that separately. We have default reports in the "Budget" section of the reporting module such as "Customer Billing" that will factor in expenses as well, which will help in charging those back to customers. You can also attach the invoices to expense sheets (or projects) so you have a physical record in addition to noting the amounts in the expense sheets.