I would love to see this happen, but only if the wall was built exclusively through the labour of the people who proposed it. The Canada-USA border is the largest unguarded land border on the planet for reasons beyond the goodwill between our nations. Features along it like the Great Lakes and the Rocky Mountains make building a wall impossible. Thankfully reality doesn’t matter to Republicans so they could be convinced to try.
Not sure about a decrease, but you can easily get it fully refunded if they fuck up. I tip really well because I used to be a food courier myself. Every few months some idiot delivers my pizza upside down or leaves my order outside my building and I get my tip refunded with ease. I have no idea if that money is refunded from the dasher’s pay though.