In the USA you currently can't email a remittance in Quickbooks online. 

You can print or download a remittance advise ONLY for a CREDIT CARD PAYMENT.  This function is NOT AVAILABLE for a CHECK payment.

 When I pay a vendor with over 50 bills on the payment, the details are too much to fit on the check slip.  Currently I have to FAKE pay it with a credit card in Quickbooks Online.  Print/download the remittance I need, then switch the payment to a check payment.  It is the only work around I have found for now.

Quickbooks really needs to add this feature.