You can change the subject lines of the emails that Shopify are sending to your customers, from within the Shopify admin:
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In your Shopify admin, go to the Settings page
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Click on the "Notifications" section

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Find and click on the email notification that you would like to change the subject for, for example, the "Order confirmation email"
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Update the field "Email subject" (see screenshot below)
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Once done, click "Save"
