How to add metadata to Stripe receipt emails?

I'm trying to add some data to the transfer() function, but I'm getting these errors:

Undefined array key "customer_firstname"
Undefined array key "customer_lastname"

I added the following line to the $request array:

'metadata[Customer]'  => $order->data['customer_firstname'] .' '. $order->data['customer_lastname'],

Is this not the correct field format?

$order->data['column_name']

Or maybe it's

$order->data['customer']['firstname']

Also, how would you test this module on a live store without placing an actual order?

It's $order->data['customer']['firstname']. Consider doing var_dump($order->data) to see the variables and names.

how would you test this module on a live store without placing an actual order?

Stripe have sandbox account for this. As for test orders in LiteCart, I recommend a dev platform.
https://www.litecart.net/en/knowledgebase/8/what-are-10-things-everyone-should-learn-before-using-litecart