You're welcome! Bummer that the table isn't having fun with the current level of challenge though. The game is designed to be pretty over the top, kind of "here for a good time, not a long time", so super lengthy "traditional" campaigns may not be the best fit as-written right now (I see the campaign length as closer to 3-6 sessions, but each table is going to be different). I don't want this to come across as dismissive or anything, but right now the game is functioning as-designed (though I will admit the sale stacking issue is something that I completely overlooked😩), so there's only so much I can offer as suggestions!
That being said, there's nothing stopping your table from changing things within the game to try and align it better with what you're looking to get out of it. Off the top of my head, the easiest elements to adjust are enemy HP and the cost of items, if you're looking to draw out fights and make buying things more challenging. You can also increase the narrative stakes, but that's going to depend entirely on what's happening at your table, so I can't offer much in specific examples there unfortunately.
As for the Metallic Shout bonus, I think it's still fun! But if it feels "weak" you can also push what exactly "rip and tear" means. Why stop at something physical? Maybe with enough juice, a Player can rip and tear through the veil that separates the City and the Inversion itself? The effect is as strong as however you choose to interpret it.
I sincerely hope that you can find the fun you're looking for in the game and I'm more than happy to keep discussing ways to tweak things to suit what you're looking for!