I had similar issue two weeks ago at GPO City Square (parcels). I had to pay taxes on a something I won.
So I complained a lot and had to see "the big boss" who told me, after a nice welcome "Sema Mzungu" (grrr..) that awards and prices are taxable; same applies to gift with are taxable under valuable gift.
Customs tax are 25% of value and VAT is 16% of value. So, they can't charge you more than that !!
If you are not happy with prices to be paid (by customs officers), you can ask to speak to "the big boss". So I paid a second visit to thebigboss who welcomed me again with ("so what now mzungu don't want to pay tax"). From what I have seen, he is spending his days on Google Shoppping, Amazon and Ebay. So, I ask him to check the value of the item on Amazon and set price based on current market price; which he did within a minute.
So to make it short, disagree, ask to see the boss, search on the web current market price and make sure you are charge 25 % + 16 %.