Here is the problem.
The customer adds a product (quantity x1)
When you check the shipping rates everything looks ok: https://www.billboardvinyls.com/cart/shipping_rates.json?shipping_address[zip]=35242&shipping_addres...
The customer changes the quantity x6 and Cart API can't generate rates anymore.
With the same quantity(x6) everything is fine at the checkout.
I believe this is a small bug but Shopify support redirects me here to get some help.
Any help would be much appreciated.
Are you doing this?
The recommended method to generate shipping rates is to use the POST /cart/prepare_shipping_rates.json and GET /cart/async_shipping_rates.json endpoints because it might take a while for shipping rates to be returned. The GET /cart/shipping_rates.json endpoint is subject to throttling
Without knowing what calls or frequency you're making them in perhaps you're seeing a throttle. See how it goes with the async options and if that yields better results first.