So, I've answered my own question! Thanks to FastSpring support for providing the variable list, which I'm copying here for future LimeLM devs that use FastSpring. Here are the standard attributes that FastSpring collects on orders in PHP:
Parameters$name - This may be the customer's full name, company name, or email address, depending on License Name and the customer's choice.$name_unicode - Array of Unicode codepoints (integers) representing name.$company - Customer's company, if entered.$company_unicode - Array of Unicode codepoints (integers) representing company.$email - Customer's email address.$quantity - It is the responsibility of the script to determine how quantity is handled. If you return multiple licenses, all licenses will be assigned to the order.$product - The product name.$reference - The order's reference / ID.$subscriptionReference - The subscription reference / ID.$tags - Map of tag names to tag quantities.Tag names are defined at the product level and allow license generation to be based on other products in an order. Tag values are always 1 or greater.$referrer - A custom value passed into the order process via the URL.
$name will return email if they have Products and Pages -> (product) -> Edit (License Fulfillment) -> License Name set to Email Address. Change that to Person Name, and $name will give you full name and $email will give you email.
A single script call is processed regardless of quantity -- it is the script's responsibility to return multiple licenses if applicable. The following global values will be visible to the script.