Low ball the value will mean they will have to pay less taxes - we have a Federal Tax called GST it is 5%. Lowballing the item is not illegal. Marking it as a gift (which would be excempt from the 5% tax) is.
Any courier can be hit and miss - they can charge as much as 20% 'brokerage' fees. I was shipped a $200 item once , Puralator - had to pay $48.00 fee
USPS is the best method - usually only get hit with 5% GST and a $5 - $10 'administration' fee.
I assume you will get the money first - so you are covered. If you want to insist on insurance for replacement value - then I would not lowball the cost.
Sometimes things get 'stuck' at the boarder - somethimes not. Usuallt pretty good , but it can happen.
My advice - mark it as true value , insure it for such and be done with it. Good luck.