Feel free to link to data on the effects of sales tax on the price elasticity of toaster ovens if you'd like to scratch that particular academic itch but as unofan has pointed out, none of that changes the fact that sales taxes are regressive. As in period, end of story.
With no data of my own, just common sense and personal observation, I find that assertion ludicrous. That 5% is just tacked on, market forces/competition should, and undoubtedly would, drive that out of the price. Or are the laws of supply and demand no longer a thing in economics?