My understanding is that under-declaration of value is not well-regarded by authorities in most countries and, if detected and determined to be an attempt to evade dues, all manner of unpleasant things may ensue, including confiscation of goods. It is my experience that some non-EU suppliers expressly state that they will not under-declare.
It is, of course, a moot point as to what the actual declarable value is, and upon what it is based.
It's mainly luck whether something declared at a value attracting VAT is stopped and a charge raised to the recipient. I've had very expensive razors arrive without trouble, but I recently bought a pen from the USA, which, with shipping, was $29 (about £22 UK), and I was deeply p****d off to be charged about £4 VAT plus the £8 RM fee on top.
The other thing I've had trouble with, is the Customs people often don't seem to know the difference between Canadian and US dollars, and misvalue the shipment. I see today that £10 UK = $12.87 USD, or $17.65 CAN. That makes a difference.
I agree with
@chris.hale about buying stuff in the UK when it gets here. The downside, which I've found going back to buying minicomputers and mainframes in the 1970's, as well as latterly, is that whatever was priced in the US in dollars magically turns into the same in pounds sterling. That's not so significant with the present exchange rate, but it wan't too good when the rate was £1=$2 + and thereabouts.
Speaking personally, if there is something I want, and can't buy elsewhere, then I'll get it, and factor in the likely on-costs to make sure I'm in budget. The heartburn for me mostly arises from the vagaries, weird transits, black holes etc. which happen with USPS and others in the delivery stage between the US and UK, and then the equally odd meanderings of my package through HWDC, HMRC and Royal Mail/Parcelforce.
