We (2 adults) need to fly one-way SCL to PUQ. I haven't checked the price on the LAN.cl site yet, but found the round-trip fare on the US site is acceptable.

Is there any problem with booking the RT and not using the return?

We actually have the one-way flight booked with Avios, but it is the afternoon flight that makes a stop on the way. This was the only one offered when I booked, and still the only Avios flight that makes sense, time-wise. It is worth it to us to pay around $US400 to fly non-stop--- I am not a good flyer and especially hate taking off and landing. As a bonus we would get there earlier in the day and have some time to look around.