Item IDs, tax, shipping, and discount amounts in payment session

I’m trying to integrate “Snap! finance” as a custom payment gateway. Their API requires the Sku/id for every product, and the tax/shipping/discount breakdown for the order. Currently, in the payment-session the Sku/Id for the products is not available and there is only a total amount without shipping/tax breakdowns. I can see this information in the orders API but they are not available in the payment session. Is there a way to get these values in the payment session before creating the order?