@Bridemom If it helps, my wedding cake (3 tier with intricate buttericing, which cost £500) fell over in transit on the way to my venue. The cake lady hit a pothole (which my dad broke to me, with a very grave face as saying 'Sam, there's been an accident', so I legit thought one of my guests had died en route).
As a result, the top two tiers weren't salvageable. The cake lady, sobbing due to my mum's tongue lashing, spent the ceremony icing two dummy tiers and trying to re-ice the bottom, whilst the venue and a local bakery churned out victoria sponge for my guests to take home. I got 50% discount.
Without my bride head on, I probably would have asked for more, but at the time, I didn't give a fudge about the stupid cake falling over - everything went wrong on the day, but none of that mattered. Ten years on, it's all just an amusing story.
Obviously, I can see why you were unhappy your cake was less than perfect, but if it was hot, a buttercream cake doesn't stand a chance. In my opinion, the venue was responsible for leaving it out; it should have been wheeled out for the cake cutting, but otherwise refrigerated. I don't think it's reasonable for the cake maker to make a loss - she wasn't in control of the weather or the venue keeping it out for too long, and you did get to eat it, at least - so a third of the cost is generous. If you'd be happier with two thirds, maybe see if the venue can reimburse you the rest for leaving it out. Honestly, I'd be annoyed at them.
I hope otherwise you had a lovely day... and here's a picture of my patched up cake (which wasn't how it was supposed to look). along with our grooms cake - Audrey Two :)