Update: Thursday 3/28 9:30pm EST
After some additional investigation, we learned that our default shipping rate generator that is integrated with BigCommerce had API issues which needed repairing. Those issues are now fixed
I've switched back to our default shipping service to generate shipping rates, and made adjustments to the login parameters that should remove the error message customers were receiving at checkout. It seems that the issue was not actually Paypal, instead it had to do with the shipping parameters not properly returning values after checkout. We had this with our 3rd party shipping rate service, but switching back to the default service required setting up new parameters that we weren't aware of until speaking with BigCommerce support.
If you are still seeing issues during check out, please send a screenshot to firstname.lastname@example.org. We will continue to work with BigCommerce engineering to rectify software issues so that promotional shipping rates such as FA Economy are returned to the site.
I heartily thank you for your patience as we worked to resolve.
We're receiving reports today that checkout is impaired when using Paypal, and affecting numerous stores. You may see this message should you choose to checkout using PayPal:
AN UPDATE TO YOUR SHOPPING CART HAS BEEN DETECTED, AND YOUR AVAILABLE SHIPPING COSTS HAVE BEEN UPDATED. PLEASE RE-SELECT A SHIPPING METHOD TO CONTINUE.
Internal attempts to replicate the checkout issue were unsuccessful, as we were able to complete checkout, and some customers using Paypal have also completed their order.
We are investigating with Bigcommerce - our e-commerce partner - now and will post updates as they occur. Thanks for your patience.