I'm not aware of a way to do this in the US, or else I'd absolutely take this on myself. For JLC the classification code is chosen by me at checkout, and I use the industry standard code for the product I sell.
Export or import classification? The first six digits are the same, but the rest vary per country so if you specify the export tariff codes then the one doing the import declaration will still have to do the import classification.
Haven't had to classify when ordering from JLC, then again I've only ordered PCBs so that might be why.