Then you can get another 2 pairs of these priced at just $5 each.
That will put you at $25.90 for 4 pairs of sunglasses. So you can pay $2 for your voucher and then a whopping 90 cents after your voucher. That is a great price for 4 pairs of glasses even if you still had to spend $12 on the voucher . I am sure you will find a ton of scenarios that will work out great but this was the first one that came to mind.
I also found these glasses priced at just $3.95