Is there anything special about Custom Segments at the Line Level that needs to be accounted for when attempting to populate via RESTLet? I'm currently passing the custom segment id: cseg2, and the id of the list value. However, it's not taking on the import. That field is being left blank. TIA!

Stretching my memory, but I'm thinking there's actually a different field ID for the line item field and it's not immediately obvious. Like if your custom segment is customrecord_yourvalue, the line item value is something like customrecord_yourvalue_line. I could be totally off on this, but a quick way to check would be to just hand-key a transaction in a Sandbox that's close-ish to what you're attempting to do via the RESTlet and fill in the line item value. Then use the field explorer extension or something similar to see what the field ID is that's being set.

make sure you've set the permissions on the custom segment at the role level and on the custom segment itself