Another EU import query - charges under the £135 threshold

[Complex]

Clearly this is a topic that comes up a lot, though from my searches it tends to be queries on what charges to expect over the threshold:

I ordered £121 (total excluding shipping, as confirmed on the import docs) worth of car parts from Denmark. I understood that any charges on VAT should be taken at point of sale as it's under £135. I have previously ordered from the same place with no additional import charges when under £135. They're a large (mainly B2C) parts distributor who export all over.

The parcel arrived one week ago and I have just received a bill from FedEx, along with a stack of supporting import docs for:

£24.20 in VAT charges for the goods (Code B00)
£20.00 in deferred handling fee from FedEx, due to the above outstanding VAT

Presumably this happens when the seller has not dealt with the UK VAT at point of sale. My question is whether if this correct and if not what can I do about it? I have not phoned FedEx yet to query as I don't think the issue lies with them but wanted to see if I have a leg to stand on against the supplier?

Very grateful for any help - would be a shame to be put completely off buying from the EU but this roulette on charges seems BS