laravel Adding shipping rate to checkout session results in "invalid array" exception
//Right now your code is just passing a single hash for shipping_options instead of an array, so instead of this:
'shipping_options' => [
'shipping_rate' => [env('SHIPPING_KEY')],
],
//you need to move the brackets to look like this:
'shipping_options' => [
['shipping_rate' => env('SHIPPING_KEY'),],
],