Well,you shouldn't worry much about paying with one or other currency,since the conversion is automatic,just make sure you have enough funds to pay at any given time.
However,the conversion rates is something you definitely want to pay attention to,since they're not the same for every site you can buy from and are constantly updated.
You can check the currency converter option here to at least have a reference,but always make sure the quantities are correct during the check out process when you pay while buying any product/service online.
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