[blunte]

If the value of bitcoin goes up relative to the local currency (CHF, EUR, USD), then when you pay next month, the amount of BTC you'll need to pay will be lower.

They are going to charge you whatever amount of bitcoin gives them X CHF per month. If bitcoin doubles in value relative to CHF, then next month you'll probably be asked to pay half as much bitcoin.

This is no different than if you were buying service from a foreign company, and that company charged you in their local currency. You would have to pay the current equivalent in your local currency. Sometimes you would pay more than last month, sometimes less.