Whilst i believe Amazon technically doesn't allow BOGOF promotions for merchants there is a work around..
Simply add 2 as a quantity to the shopping cart on checkout. Then apply the promo code in the 'Enter gift card or promotional code' section. This will bring the total price to $27.99 for the 2 pairs of sunglasses.
These a good Category 3 polarised lenses that have been tested to the new AS/NZS 1067.1:2016 standard.
Currently 41 left in Amazon FBA warehouses.