Free download with purchase?

Hello,

I haven't been able to find an answer to this one:

I'd like to offer a free download with any purchase from my store.
I am a music company, and I sell CDs, MP3s, Ringtones, clothing, etc.
I know I can add the download as an attribute to my existing products, but this poses two problems:

1. There are too many products! Adding the download to each individual product would take an awfully long time.
2. If a customer buys 2 or 3 different products, they will get the same free download 2 or 3 different times.

I'd just like for the one download to be added to their cart upon checkout.
Any ideas as to how to achieve this?

Thanks!
Bob

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