Chargeback LOST despite valid proof of delivery. Any suggestions?

[nyclinda]

I sold a dress to a customer mailed via USPS with tracking.  The item was delivered but the customer claimed she didn't receive it.  I sent her the tracking details as backup but she still claims she didn't receive it and opened a chargeback.  I contacted USPS and they emailed me confirmation that the tracking was correct and their gps confirmed the delivery.  

Today I received the shopify notification that the chargeback was decided in the customers favor.  They provide NO DETAILS on why.  So then I contacted American Express (the bank) who said they can't help me because my transactions go through Shopify payments powered by stripe.  They said if Shopify/Stripe called their dedicated payment processor customer support number, they would be able to revisit the claim and stated if your proof is valid, this sounds like it could be reversed. 

Shopify says they don't do that and they won't reach out to stripe on my behalf either. Shopify said they only act as a platform to share information. 

 I am SO frustrated!

Shopify support is useless and AMEX can't help me without Shopify reaching out to them.

Any suggestions on next steps or is it a lost cause?

Thank you.