I'm currently studying the TCP layer, one of the main references is the 1981 Internet Engineering Task Force's RFC. One of the paper it cites is a paper published by the Internet Electric and Electronic Engineers in 1974.
In the 1974 paper, one of the considerations for the design of the protocol seems to be the attribution of charges, accounting of packet costs, billing:
To allow networks under different ownership to interconnect, some accounting will undoubtedly be needed for traffic that passes across the interface. In its simplest terms, this involves an accounting of packets handled by each net for which charges are passed from net to net until the buck finally stops at the user or his representative.
Later, this rationale is expanded and curious details about the expected billing
We assume, for simplicity, that each network initially charges a fixed rate per packet transmitted, regardless of distance, and if one network can handle a larger packet size than another, it charges a proportionally larger price per packet. We also assume that a subsequent increase in any network’s packet size does not result in additional cost per packet to its users...
The assumption of simplicity is that charges don't depend on distance travelled, which is surprisingly true nowadays. But the charge per packet is taken as a natural assumption, and the implications of it are further discussed in the paper.
There are 2 oddities here.
Charges are measured in packets instead of bits: Nowadays, almost all ISP contracts are stipulated in terms of bits, not packages. Was there ever a time were charges were calculated by the amount of packets sent or received?
Charges are measured in data usage not bandwidth : This is the most important difference since, nowadays, most consumer's traffic is billed by the bandwidth, although some large scale hosting providers might internally account per data usage, and mobile providers may charge by data usage. I mention this difference because the original article speaks of stopping the buck at the user representative, which implies that the user is charged per data usage, not bandwidth.