Via Alexa-enabled devices and for Prime members only, Amazon offers a $5 Amazon Gift Card for free when you buy a $25 Amazon Gift Card. (Say "Alexa, order a $25 gift card" to get this deal. The credit will be delivered via email within seven days.) That's essentially a free $5 on your gift card total. Some exclusions apply and it's limited to one gift card per customer. Deal ends April 16.
Of note, you can instead reorder an item from your history to get a $5 credit. As above, this credit is limited to one per customer.