The tour to the Humantay lake is offered by many travel agencies, which offer the service through digital channels (web pages), or in establishments near the historic center of Cusco.
We recommend that you take the use of formal tourism agencies that guarantee the tour and have a backing of state and tourism entities such as: Sunat, Sunarp, Mincetur and Gercetur that guarantee that the tourism travel agency has the backing of mentioned entities, therefore any claim regarding the service you can do it without any problem, the informal agencies is very difficult to make you a claim since they are not authorized and do not have a backing of tourism entities.
Ready to Travel Peru is a tourism agency that has all the permits required by law, and is also registered in the service of qualified tourism agencies by the Ministry of Commerce and Tourism (Mincetur).