The payment method defaults, that you defined in your payment settings can be overridden for individual customers.
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For example, if you are running UK based business and collect your recurring membership payments by Direct Debit, but then have a customer join from abroad who hasn't set up a UK bank account yet, then you could allow card payments just on their account. You may have long-standing customers or friends that want to pay cash.
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💡 Check your payment method defaults under Settings >> Collect Payments