A Customer Lead is a potential customer and does not sync to QuickBooks.
Converting a lead to a customer will remove the lead status and the customer will immediately sync to QuickBooks from Method.
There are two situations where a lead is AUTOMATICALLY converted:
If a lead has a transaction (such as an invoice or an estimate), then that transaction has a checkbox that says "Wait for sync approval?" When this is unchecked and saved, the transaction and the customer will immediately sync to QuickBooks. No warning will be given.
If an Opportunity is associated with a lead, and the opportunity is saved with its Stage set to Closed Won, then the lead will be converted to a customer. You will, however, get a prompt to verify the conversion.
To MANUALLY convert a Lead to a Customer: