Correct delivery estimates when a discount is applied

Shipping eligibility at checkout is now calculated on the cart total after discounts. Previously it used the line-items total, so a basket with a discount code could show shipping methods it no longer qualified for, or a delivery date that didn't match the order.

This applies to stores using Bigblue's carrier-calculated rates in Shopify. Custom or flat shipping rates bypass the calculation, and stores with multiple shipping profiles aren't supported yet. ETA at checkout must be enabled in your Shopify shipping settings.

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