Yes, it can be added to any reward except the "$43 3 Large Packages" just add an additional $10 to your pledge and we'll know to include it. (The $43 reward is designed for a specific weight that cannot be added to.)
5 packages is correct. As you may have guessed, it was a classic cut and paste mistake. The first sentence should read. "We'll send you five packages of 20 sheets allowing you to create 600 of your own cards." (It just goes to show you that even when 2 separate people proof read the work mistakes can still slip by!)
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