You can select the following fields to export the shipping refund order adjustments (note that these values are negative):

• refunds.order_adjustments.shipping_refund.total_amount - The total amount refunded for shipping (excludes taxes).
• refunds.order_adjustments.shipping_refund.total_tax_amount - The total tax amount refunded for shipping.

You can add them together using Calculated Fields:

{{ refunds.order_adjustments.shipping_refund.total_amount }} + {{ refunds.order_adjustments.shipping_refund.total_tax_amount }}

If you want to convert the value to a positive number, simply multiply by -1:

({{ refunds.order_adjustments.shipping_refund.total_amount }} + {{ refunds.order_adjustments.shipping_refund.total_tax_amount }}) * -1