I believe Battlestamps is correct. The postal markings seem to indicate that the enclosure refers to postal business and was mailed from Post Office 1 in Vienna. Enclosure was probably a bill for the recipient.
I believe the cancel is one publicizing the donation campaign for the WHW charity. During this time, various Nazi groups would go out soliciting donations at various times, therefore "street collection". There are many reports of retaliation against people known to have not made a donation or did not donate what was considered enough. You may be aware that the extra charge from 3rd Reich semipostals went into officials pockets and not to charity, so one might assume the same happened with this campaign generally.