HMRC added value to my imported package

Hey every one,

Recently I've imported goods from china into the UK. Total paid was 67.57£ . As I want to set up a business (fashion super cheap jewelry) and wanted to do everything by the book I've declared the exact correct amount paid.
But then DHL informed me that I need to pay 32.48£ fees.
Charges breakdown:
Supplier's invoice value:67.57£
Freight value:33.61£
Value for customs purposes:101.18£
Vat value adjustment:6.25£
Value for VAT purposes:107.43£
VAT: 21.48£
Advance payment:11£ (I don't have problem with that I know what this charge about)
Total :32.48£
I've contacted DHL and they explained me that the extra under "freight value " and "vat value adjustment" is something that HMRC added.
Tried to get help from HMRC but their tired customer services lady just told me that HMRC can add value if they think that it been under-declared.
Does any one know something about this "VAT value adjustment?"
As my bank transactions can prove that I've paid the right amount how can I get refund from them?
I'm worried bcs this package is just the samples delivery and I was hoping to order bigger delivery in the future. And if that something that will occur again I need to know how can I deal with HMRC and what I can claim back.

Any answer would be appreciated ,many thanks in advance.