Guest submission journeys

I'm dealing with a client who's looking into examples of companies who allow users to submit data to them without logging in. They are looking at receiving invoices from multiple members without them needing to log in.

For example: Royal Mail allowing users to create pre-paid labels for "Send-from-home" parcels etc. In many cases a purchase is necessary, in others there is an authentication layer.

I'd absolutely appreciate any examples, help or experience with this type of UX — they can take any form and be across any industry.

Thanks.