What are some of the best and cheapest point of sale systems available in Canada?

Easiest and best way is to have shopify POS with website and start selling same products online as well. otherwise you might buy separate website and pay for it. i suggest whatever system you use then add Unique Barcodes to your store as unique identifier. also you can buy upc numbers from

https://codesforamazon.com/product/us-upc-codes-for-usa-marketplace-at-amazon-de-ebay-de-copy/

As many time we have items without barcode and we messed with invalid codes so whatever system you will use buy from here to use in your system so your item will be accpetable at amazon ebay and shopify website to sell online.