I know of that device, I've seen it used by customs in various videos on YouTube, but are you sure it can detect PMK ethyl or mehyl glycidate? Not all packages in customs are inspected, it all depends on several factors, so it is luck more than anything, also in Alibaba even if the Chinese seller changes the name of the chemical on the invoice, if you make the payment through trade assurance , there will be little chance of being scammed, since you can open a dispute against the seller and ask for a refund.