It's happened to me twice now, just ring them as you have done and ask for a specific invoice from the inspection officer for the particular parcel you were sent, that indicates that the fee has in fact been paid in full by DHL for that parcel - they can't provide it, they will tell you some story about the stamp on the shipping docket being proof of inspection and payment by DHL (BS), as others have said, fee will be waived.
Often the invoice can turn up weeks after you have received the parcel, nothing they can do about it if you don't pay it - hey, you have already got your gear.
I have found that if the shipping company makes reference to "lures", "fishing gear" or "timber" in any way - it can attract the eye of the inspectors, better if they can simply tag your parcel as "Gift". If that is not possible, make sure they state "NEW" before words like "Fishing Gear", you can't blame the average customs officer who knows SFA about our sport, for wanting to inspect what they probably visualise as 'dirty', 'smelly', 'old', 'disease riddled' fishing gear!