What could cause bubbling in the downstairs toilet of an urban condo?
I'd start by snaking the shower drain. Liquid drain cleaners help with grease and oil buildup in the trap, but less so on hair or other solids, especially if they are further down the line (e.g. near the vent). The shower drain will be a lot easier to snake since you don't have to worry about damaging the toilet's porcelain finish.