Hello, it seems that discount code only applies to registration cost, but not to cost generated by the feed fields. Please confirm if this is the case.
It can work either way. In the list of events (backend) you'll see "Fee Ordering" to the right. Click that and YOU set the ordering of when your fees are calculated. This includes base fee, discount codes, fee fields, etc. If you want your discount codes to apply after everything else, then drag-n-drop the box representing your discount code(s) to the bottom of the list.