How to send conditional mail confirmation after completing payment using webform
You could use the CiviRules extension (https://civicrm.org/extensions/civirules) and the E-mail api extension (https://civicrm.org/extensions/e-mail-api).

You could setup a rule which is triggered when a contribution is added and add a condition the payment instrument (and probably also the financial type). You could then add an action which is send e-mail (this action is provided by the e-mail api extension).

Documentation of CiviRules is at: https://docs.civicrm.org/civirules/en/latest