This is exactly what I was looking for

This plugin does exactly what it’s supposed to. I can specify a single or multiple gift skus for each product on my site. The free sku is added to the cart automatically, no annoying pop ups. If the paid sku is removed from the cart – the gift sku remains, but is charged full price. Product page and Cart page look great – customer knows exactly what they’re getting. When I first installed the plugin there was a small problem with variable products, but the plugin author addressed the issue very quickly and now it works perfectly.

In the past I used a different plugin, but it never worked as smoothly as I would have liked. This is a simpler and much nicer option. I highly recommend this plugin.