Hi there,
In the Czech republic, you can typically withdraw the equivalent of CHF 300 in a single ATM withdrawal. This means you only pay the foreign ATM withdrawal fee once (this fee ranges between CHF 4 and CHF 5 at Clientis banks, with some Clientis banks adding an additional fee as a percentage of the amount withdrawn (at Clientis Zürcher Regionalbank the fee is CHF 5 plus 0.5%). The forex rate is generally used for foreign ATM transactions. You will not normally pay a third-party ATM fee as long as you use ATMs operated by genuine Czech banks and not by non-bank ATM operators. Depending on the Clientis bank that you bank with, you can expect to pay around CHF 5 in fees to withdraw the equivalent of CHF 300, in addition to the money you lose on the exchange rate spread (you can request a comparison of Clientis bank spreads here.
Using a credit card to get cash advances at ATMs is not recommended because you will pay a cash advance fee in addition to the foreign transaction fee. The cash advance fee of the Migros Cumulus Mastercard is 3.75% (min. CHF 10), and the foreign transaction fee is 1.5%, meaning you will pay a total of CHF 14.75 to withdraw the equivalent of CHF 300, in addition to the cost of the exchange rate spread.
If you change money at the till of Clientis, you will get a less favorable banknote rate. However, the absence of a commission (most Swiss banks waive commissions for account holders) means that you can still get a good rate, depending on the exchange spread used by your specific Clientis bank. In many cases, you get a better deal by changing CHF to EUR and then changing EUR into other currencies (particularly less widely circulated currencies like the Czeck kronor) at the till of your Swiss bank.
Another option is to use your debit card to pay for large charges directly at POS terminals, if and when this is possible. Swiss banks typically use the more favorable forex rate for debit card transactions and the foreign transaction fee (typically a flat fee of CHF 1.50 per transaction) is relatively low if used for large payments.
Best regards from Moneyguru
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Swiss banknote rates and forex rates explained