Hi there,
You should choose to pay in euros when shopping at Amazon.de. The reason for this is that data which we have collected shows that Amazon adds a markup of 3% - 4% above the Interband rate when changing Swiss francs to euros.
In the worst case, you may pay more than that to your credit card issuer in foreign transaction fees and exchange markups. However, if you use the most affordable credit cards for foreign transactions you will spend less than you would by paying in Swiss francs using Amazon's dynamic currency conversion. You can find the best credit cards for foreign transactions using the interactive credit card comparison which also accounts for CHF - EUR exchange rates.
Another factor is that many Swiss credit card issuers charge foreign transaction fees for all purchases from foreign merchants, even purchases which are paid in Swiss francs. When you use credit cards from these issuers to pay on Amazon.de, you pay a foreign transaction fee whether you pay in euros or Swiss francs. When you pay in Swiss francs you pay both the credit card foreign transaction fee and the dynamic currency conversion fee from Amazon.
The info pages of cards included in the credit card comparison clearly state whether foreign transaction fees apply to all purchases from foreign merchants or only to purchases paid in foreign currency.
Best regards from Moneyguru
More on this topic:
Credit card foreign transaction fees explained
Dynamic currency conversion explained
Guide to avoiding dynamic pricing when shopping online