If anyone took advantage of the 99p Amazon dash buttons yesterday, you can use the £4.99 discount on anything, not just against the product that would be automatically ordered on your first button press.
Just press the button to place your first order, then cancel it.
The £4.99 will be added as a promotional credit on your account to use against anything you want. Just add to your basket, and place the order.
(Note: It doesn't apply it as a gift voucher credit, it adds it as a promotional balance, so you only see it if you have something in your basket).