Mode of payment by any manner is valid including online, if the payment acknowledged for the purpose made that is sufficient proof of the service provided to the customer even if no bill generated or given for the service by the service provider.
All these are not at the client's request but depends on the Advocate's demand and mutual understanding. Except for Institutions, any advocate mostly demands fee only in cash irrespective of the amount. I have not even heard the practice of giving receipt by Advocate. Everything depends on mutual understanding. In case of unfortunate demise of such an advocate who has collected full fees even before filing plaint, there are complications as the fees received was within the client and his advocate only.