Some accept visa cards, others don't, check before you visit. Generally speaking it is advisable to contact hut guardians to assess the need for pre booking, food availability, etc before arriving. Phone guardian direct or tourist info in the nearest valley. Most hut wardens supply several food options. Custom varies, in France it is common to take your own food and cook it yourself in or outside the hut.
Most wardens prefer gas stoves inside the hut.
In Switzerland most guardians require that you give your food to them, they will then prepare it. Most huts offer meals, these are usually good if expensive. Buying hot water in the morning can save precious sleeping time and reduce the need to carry stoves and fuel. Wild camping is officially frowned upon. Bivvy's however do traditionally exist in certain areas from sunrise to sunset.