Add 2 pairs as a quantity to the shopping cart on checkout. Then apply the coupon code in the 'Enter gift card or promotional code' section. This will bring the total price to $29.99 for the 2 pairs of sunglasses.
There are still 30 or so at Amazon warehouses. They have good Category 3 polarised lenses that have been tested to the new AS/NZS 1067.1:2016 standard.