It's better described as evidence of a contract. But I guess a signed piece of paper is merely evidence of a contract as well, despite being commonly referred to as "a contract".
But this isn't really relevant when discussing healthcare, because the problem is precisely with providers not dealing in specifics that would be necessary to create a printed receipt. It would be a terrible system if hospitals demanded full payment ahead of treatment and price gouged patients on the spot, but it would be significantly more legitimate than what we currently have!
