1) Login to your Shopify Admin.
2) Click "Checkout".
3) If the PayPal Express had been automatically setup by Shopify, please click "Edit" and click "Deactiveate". The auto setup didn't grant permission for payment refund.
If nothing setted, please click the pulldown menu under PayPal and choose "PayPal Express Checkout"
4) Click "Activate"
5) It will redirect you to PayPal login page.
Login to your PayPal account.
6) Click "Grant Permission"
7) It will redirect back to Shopify admin and showing "Using PayPal Express Checkout"
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