This promotion is good until July 31st (usable once per Amazon account).
https://www.amazon.com/b/?node=14583169011
Sign into Amazon, get your barcode, and then have the cashier at your local CVS scan it. Tell them you want to put $20 on it, and then pay with your credit card. Presto! $30 was added to your Amazon account.
edit: it took me a couple tries to get the CVS cashier to understand what I was asking of him, since many of them aren't aware of Amazon Cash. Just show your phone, say "Can I add some money to my Amazon account?" and if they give you trouble just say "After you scan it it'll ask you how much I want to put on it. Can you type in $20?" And then pay with your credit card.