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When I visited caves in the state of Nuevo Leon in north-eastern Mexico in November 1987 I did not expect to encounter any spelean post offices, and so I was unprepared for handbacks. But at Grutas Garcia, a really fine commercial cave close to the industrial city of Monterrey, I saw the word Correos at the cave's ticket window.

So I tried my best Spanish, which is not very good. But it was good enough. Not only did they sell stamps at the admission ticket window, but the administrator of the cave (now a state park) doubles as post master and postal clerk. The cancelling device is a double-circle date stamp without bars, with the word Correos at the top and Grutas de Villas Garcia below (Villas Garcia is a nearby town, for which the cave was named).

The administrator/postmaster was very cordial and ransacked his office looking for envelopes, but none were to be found, so I bought a few postcards for handback cancels - my first from a Mexico cave. I do not know if others exist or have existed in the past.

Because of current inflation which is causing very rapid rises in postal rates, obtaining handbacks by mail will be difficult.
 
Normally the mail from the state park is hand-carried to Villas Garcia or Monterrey for mailing and it had been almost a month since the canceller had been used, judging by the date stamp, so examples must be quite scarce on cover.