The threshold is actually £15, it was reduced from £18 a few years ago.
For items between £15 and £135, you get charged VAT but no customs duty, so your £3.18 suggests they valued it at £15.90.
It's hard luck that you're only just over the threshold, but unfortunately that's the way it works - and of course the £8 fixed admin fee is disproportionately high on a low-value item.
So no grounds for a claim I'm afraid. Sometimes they do get it wrong (because they've misread the value or something) and you can claim the VAT back, I've done that several times. But you can't get the £8 admin fee back.
It's swings and roundabouts, though, I've quite often escaped a VAT charge on much more valuable items, even when the seller has put the full value on the declaration.
I don´t know if this is even legal as I think goods´value need to be more than £36 to be charged at all.
That £36 threshold applied only to gifts (it's now been increased to £40). And it has to really be a gift, if it's been marked as a "gift" but it's come from Amazon, they'll still charge you the VAT!