Paypal Not too sure about IPN, when to use? -

is there need use ipn with

1. expresscheckout (setexpresscheckout => doexpresscheckout) 2. recurring payment (subscription) 

it depends on how operate business...

if ship physical goods, then, of course human check if received payment before send goods customer...

but digital goods it's story, want deliver digital content payment in account. that's main usage ipn. if ipn confirmation, fire email download link directly customer's email.

other use cases be, automate tasks, accounting, mailing, renewing subscription status, reverting need revert in case payment cancelled notification, etc.