Yes, July is the school holiday season in Brazil and Foz is underserved in terms of flights (and is one of the country's prime tourist destinations). I don't know what your circumstances are, but R$384 seems to me like an ok price to avoid a 20h bus journey, especially when the latter costs R$200 (or R$350 for sleeper service). There is nothing of interest along the way.

In terms of using miles, Azul is the only one to fly this route direct (currently 13k miles). They aren't part of any alliances but have a partnership with United. Avianca flies the route via GRU but it's 20k miles and therefore poor value.

Having said that, you can find flights for R$279 on www.maxmilhas.com.br which is an award ticket marketplace (reliable and tolerated by airlines, for now). You'll probably need a Brazilian-issued credit card to buy a ticket here.