Yes, it seems to be the a recurring problem involving Evri. I don't think it makes any particular difference whether the label was purchased from Packlink, another third party or directly with Evri.
I'm assuming what happens is something along the lines of multiple parcels getting stuck together and then an automated machine incorrectly associates the weight of two parcels with a single barcode. Then a supplementary fee is automatically applied when it isn't valid.
I would not recommend using Packlink at all, they provide no customer service. If you want to use one of the economy couriers such as Evri, purchase a label directly from their website, then it takes a (useless) intermediary out of the loop.
I believe if you want to challenge the additional fees you need to supply photographic evidence of your parcel on a scale showing the weight and with a tape measure showing the correct dimensions, or whichever is needed to disprove their claim. Realistically of course very few sellers are going to take those photos before sending the parcel. So if you don't have that evidence, I believe there is little you can do with Packlink or Evri. You might want to contact your bank or card provider to raise a payment dispute over these charges.
