Based on the response from iNat staff on this post it sounds like anything involving a fee violates the guidelines: https://forum.inaturalist.org/t/users-contacting-me-to-collect-and-ship-them-my-finds/9508 . I read over the TOS quickly and wasn’t able to find what was mentioned in the forum post, but perhaps I missed it. If your search is fruitless it might makes sense to @ them for clarification.
I do see the value of collecting and shipping for research purposes. In those situations I’d place much of the onus of responsibility on the appropriate regulations for collecting and shipping on the requestor to inform the potential collector (i.e. do the research and inform if any permits are required), but ultimately the liability would lie on the collector. No idea how that would be regulated on iNat vs. anyone asking for recreational purposes (no fee), but perhaps it’s left out to be dealt with on a case-by-case basis. I don’t think it’s an extremely prevalent issue, at least not at this point.
That’s just my two cents and in no way tied to any iNat regs, but I do see a lot of value in the potential for legal collection for research purposes.