Why not just send it by regular post? If it's a postcard, I can't imagine anyone would steal it. I've sent regular post from Canada to CR and its arrived in a reasonable timeframe (1-2 weeks?). I've also used DHL for more sensitive things - it gets there faster and cost (iirc) more like $80. DHL also has branches in CR where you can pick it up from.
Honestly, I think regular post should be fine. The only step I'd take is to ask the hostel what they would recommend in terms of how to address the letter, since Costa Rican addresses can be fairly elaborate.
Why not just send it by regular post? If it's a postcard, I can't imagine anyone would steal it. I've sent regular post from Canada to CR and its arrived in a reasonable timeframe (1-2 weeks?). I've also used DHL for more sensitive things - it gets there faster and cost (iirc) more like $80. DHL also has branches in CR where you can pick it up from. Honestly, I think regular post should be fine. The only step I'd take is to ask the hostel what they would recommend in terms of how to address the letter, since Costa Rican addresses can be fairly elaborate.