I was put off by the physical copy only as well, but I needed another account to expand my [bot] empire so I bought one anyways. Even when selecting the digital purchase it rang it up as 39.99$ so I think it still applies to digital.
To replicate my steps just make a new battle.net, log in. Then click diablo 3 on your main page and then get the starter edition. Then from there I clicked digital purchase and checked out with the $20 off. So still try it if you're in need of copies.
Expires Monday 11/26
Please note that the review period for digital purchases is usually completed within a day. However, in some cases it may last up to 72 hours. During this time, the following restrictions apply:
* You can?t use the auction house (real money or gold).
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* You can't play cooperatively or chat with players who are not on your friends list.
* Global Play is not available.
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Purchase Receipt
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Customer Name: **************
Order Date: 20 Nov 2012
Order #: 85713214
(1) Diablo? III - $39.99
Credit Card Number: ****-****-****-****
Payment method: *
Item Subtotal: $39.99 USD
Tax: $0.00 USD
Shipping & Handling: $0.00 USD
Shipping Tax: $0.00 USD
Grand Total: $39.99 USD
Sold by: Blizzard Entertainment