Strikes in Latin America -- even for essential services -- can drag on for weeks and months. Neither side wishes to budge. What often happens is that the management side eventually bribes the leaders of the labor side to end the strike, and it ends with the workers receiving little or nothing. Note that I am not saying this is the case with Serpost; only that it is a common practice south of the Rio Bravo (Rio Grande for gringos).
Unlike in the U.S., where there would be massive complaints and pedal-to-the-metal news coverage, strikes are not as big of a deal down here. If customer service suffers, the attitude is, "oh well...." It can be frustrating for people who come from the developed world but it is just one more charm of living in Latin America.
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