Blockchain info wallet API callback

Question

I am implementing blockchain.info wallet api to my node.js app. I got the receive API to work, however the transactions are extremely slow, and the callbacks likewise if they even reach. Other than that it does what i need it to: pass some custom paramaters into the callback url such as customer_id, price, amount_purchased. My question is: How do I pass in multiple values into the label, something like formatting it like this: 

{'user_id': userId, 'order_amount': amount, 'order_price': price}