Hello! When you stay at Refugio Amazonas, you will need to pay your bar bill and anything you buy from the gift shop in cash. They accept American dollars and Peruvian Soles.
If you want to buy a nice t-shirt, it may be around 40-60 Soles. A large beer from the bar (Cusquena) or a Pisco Sour is around 15 soles. So what you bring for cash in this area depends on how much you plan to drink and if you would like to buy souvenirs.
Also your guide's tip should be in cash. The recommended amount to tip the guide is between $5 and $10 per person per day. There is also a place where you can leave tips for staff (housekeepers kitchen staff, etc.). Obviously tipping is at the discretion of the passenger and not required. I hope this is helpful!
You will have a lovely time. Enjoy!