If an invoice is modified in your accounting package before it is exported by the Billx (either via the Plugin or directly via the cloud), then the latest version of the invoice is sent to the Customer.
If an invoice is modified in your accounting package after it is exported by the Billx, then the original invoice remains in Billx. We do not send any modified details through to your Customers via Billx. You will need to have other business rules in place to deal with this situation.
If you create and delete an invoice in your accounting package before your next Billx syncrhonise, the invoice will not be exported in Billx nor will it be sent to your Customers.
If you delete an invoice from your accounting package after the invoice has been exported via Billx, then the invoice will not be displayed as deleted in the Billx website. Your Customers will not receive any notifications from Billx regarding deleted invoices. Please make sure you have other business rules in place for this scenario.