If you use the same SKU for multiple products then you’d need to store what product the user selected on their side.
For example, when a use selects Big Fireworks, store that selection in a variable, and then when onTransactionComplete fires, check what is stored in that variable. If you then need to send that to your EBS you can do so but as it’s not in the receipt there’ll be nothing that prevents the user sending a different value. For example they could have selected Small Fireworks, gone through the transaction, but actually send Big Fireworks to your EBS and there’s no way for you to know it was changed as it’s not part of the receipt.