From my experience, the hosted payment page is pretty limited in terms of custom fields compared to createTransactionRequest. You can usually pass extra data through merchantDefinedData or similar fields, but it won’t automatically show up as a “surcharge” in the HPP summary, it’s more for your own tracking. If you need it to appear explicitly in reports, you might have to record it separately on your side or include it in the transaction description so it’s captured when the payment comes through.
