Once you connect erplain with QuickBooks, all invoices created from erplain will be sent to QuickBooks.
You can then manage reports and payments directly from QuickBooks.
Cancelling the invoice on erplain will void the invoice on QuickBooks. Once an invoice is cancelled, you can't re-activate it.
Note that Invoices are synchronized only from erplain to QuickBooks. If you update an Invoice erplain it will update the invoice on QuickBooks, but if you update the invoice on QuickBooks it will not be updated on erplain.
If you create an Invoice on QuickBooks, this Invoice will not be created on erplain but QuickBooks will send an Inventory adjustment to erplain to keep your Inventory in sync between the 2 systems.