Hi @sihanyuan,
Thanks for reaching out with your question. Typically, in order to use any shopping credits or vouchers, you'll need to complete your purchase first. This is because the merchant you're placing your order through will still receive the full amount, however, once the transaction finishes processing through PayPal, the credit is then applied. I hope this helps, and if you continue to have trouble using your credit, please contact Customer Services for further assistance. If needed, support options can be found at the bottom of our website by clicking Contact.
Thank you.
-Derrick
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