This is patently false, bordering on scare-mongering.
Canada does 80% of their trade with the US, who has much more power than them.
The shelves are never empty in Canada.
Every other nation in fact is largely in the same boat: Australia, New Zealand, Japan, Singapore - they all depend on partners for trade.
It works, and the shelves are never empty.
In addition the US and Canada have a customs union with smooth flow of good without lengthy border checks - AND - at the same time are free to develop their own foreign trade policies and deals.
Canada could theoretically increase their GDP and literally everyone of them would be a several shades richer immediately by 'joining the USA' but it's not exactly a movement.
Respectively, there's no reason that the UK couldn't have something similar with the EU - and for some reason there's all sorts of hyperbole to suggest somehow it couldn't work.
UK citizens voted to join the EEC, and never really did for the EU, if there was a referendum on the Treaty of Lisbon in 2007 that passed in the UK ... I bet Brexit would have never happened - there'd have been too much legitimacy in a recent vote and the leaders could not have justified a referendum.
The 'empty shelves' / 'rationing medicine' memes are ridiculous.
The EU is 80% about trade, it always has been, and very few people had existential qualms with that. There would hardly be such a thing as a 'Eurosceptic' if they focused on that.