The relationship and context IMO has everything to do with whether or not it is commanding or requesting.
In the OP’s issue, the relationship is a mutual relationship to complete a common goal of a service nature. Both are on equal grounds. Both parties should assume that any request from the service provider is a requirement to complete the service and therefore adding ‘please’ would be just polite.
To be perfectly clear, the requester should have the obligation to the receiver to state whether or not the form is a requirement to provide the service or voluntary. Again, if it is a requirement then ‘please’ is just being polite.
It could best said:
“Please sign and return this required form.”
or
“Please sign and return this voluntary form.”
The adjective ‘required’ or ‘voluntary’ provides the context to the verbs ‘sign and return’ as to whether or not it is a command (requirement) or a request (voluntary). Either way, the receiver should not be offended but would understand the importance of the form.
