In reply to Alan.T:
It's 33 CHF per person (23 if a member of an alpine club) - unmanned does not mean free as it costs a lot to maintain the place and provide gas.
Fewer and fewer unmanned huts have a box to pay - normally you take a payment slip and either pay online (if you have a Swiss bank account) or at a bank or post office once you get back down.
https://sac.zermatt.ch/verein/arbenbiwak/