If you're going to have an actual bonus round for Sale, tying it in to a shopping-related mechanic is the way to go IMO. As much as we rag on US Temptation, playing Wipeout Super Knock-Off at least earned you Temptation Dollars for the major prizes.
A while back on a game show Discord, I had this idea for a bonus round:
Your total at the end of the game is your baseline for shopping, with the same sale prices as the old show. The contestant is asked questions (more difficult than the main game ones) at $50 a pop and given the category before each one. A wrong answer gives them a strike, and three strikes ends the bonus round and forfeits any additional dollars won (essentially giving them the first level prize by default). Once they have two strikes the risk element comes into play: after hearing the category they could cash out at whatever level they were at, or go on.
I had a primetime version in mind sans returning champions for this.