If the seller understates the value of the goods on the customs export forms it reduces the amount of VAT you have to pay BUT the exporter does put themselves at risk by doing this as clearly it isn't legal so, of course, I would not suggest doing this
Try to persuade the exporter not to use USPS as they deliver to UK addresses using Parcel Force who charge another £13 or so on top of any VAT and import duty
Get them to send as a" Sample " Im sure that doesnt incur vat too I might be wrong may i just add to any tax inspectors lookin in I do not endorse such practice cant speak for Brian