There really isn’t a clean way to do an upsell post payment and add the items to an order. The only way to have the payment itemized is to pass in an order. There isn’t a separate tax object that can be attached to a payment.
If you store the card on file from the first payment you can use the card on file to programmatically pay for the upsell.
I don’t need to add an item to an order necessarily, I can send it as a second order.
What is the best practice to send an order with a payment done through the SDK and have taxes applied to it? I don’t see the explanation here Pay for Orders
Do you first create the order, and then charge the card?