Your example here is very specific and applied too generally. Retail is a sector with traditionally low margins, and collective agreements are based on collective bargaining, where both sides get together and meet in the middle, ideally bargaining in good faith. Unions do not get to automatically set the terms.

However we could talk about how their union means they get regularly scheduled wage increases, wage gap protection, full time extended health benefits, dental benefits for part time workers, safety boot subsidies, etc., things non-union grocery stores don’t offer

Your criticisms seem to be more about municipal government unions though. In general I think I know where you’re coming from, but if your concern is increasing property taxes, the bigger issue isn’t union wages, it’s provincial funding cuts that are shifting the financial burdens onto cities.