You are doing everything wrong to begin with. If you dont want to use the users paypal email which is already included in transactions you should have your site query for any needed info during the checkout process on your website, paypal is a payment process not a checkout process and all info needed should already be known at that point. Encode information collected into the "custom" field so it can then be passed back and forth between paypal and your website. You should be using a store for your website that simplifies all of this and there are many free ones available for all the major CMS's
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