## Issue:
500 error on shipping/calculate and orders/preview endpoints
Error: "Call to a member function empty_cart() on null"
## Root Cause:
WC()->cart is not initialized in admin/REST API context
Calling WC()->cart->empty_cart() fails when cart is null
## Solution:
Initialize WooCommerce cart and session before using:
```php
// Initialize if not already loaded
if ( ! WC()->cart ) {
wc_load_cart();
}
if ( ! WC()->session ) {
WC()->session = new \WC_Session_Handler();
WC()->session->init();
}
// Now safe to use
WC()->cart->empty_cart();
```
## Changes:
- Added initialization in calculate_shipping()
- Added initialization in preview_order()
- Both methods now safely use WC()->cart
## Testing:
- ✅ Endpoints no longer return 500 error
- ✅ Cart operations work correctly
- ✅ Session handling works in admin context
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