It depends on what you want to do. The EP that comes with your package should be sufficient to customize your package at a basic level. If you want additional items, or more rare/higher quality items, you can purchase additional EP. Caspian put out a post on what we can expect to see in the EP store last month. The information is available on the wiki here: https://chroniclesofelyria.gamepedia.com/ExpositionPoints#SpendingEP, which also has links to any posts the Devs have made about EP.
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