Whoa now, for international flights LAN has to abide by the Montreal Convention about lost and delayed baggage which says nothing about receipts, it is a complex formula based on weight and Special Drawing Units. If lost on a domestic flight they may use the rules of that country (but I think the Andean Aeronautical Convention mirrors the Montreal Convention). Not an expert me, you need to research this, but clearly an airline will try to get you to agree to something that may be less than what you are legally entitled to. Your photos will be a powerful tool.