Why can't customers apply coupon codes due to invalid shipping rates?

[adamlitt]

Our customers experience the following error when entering a coupon code.  

“Your chosen shipping rate is no longer valid. Please select another rate.”

This has been going on for three weeks now and Shopify, our multivendor app developer and an independent 3rd party developer cannot resolve!  If you have ANY thoughts on this, please let me know!  Thanks so much. 

[Anindita]

Hi @adamlitt 

You can contact Shopify live chat support to resolve this: https://help.shopify.com/en/questions#/contact/chat