If we were thinking big picture, would this mean that the US gov't could basically spend $72 billion a year to zero out the carbon contribution of America?
Not really, because there's an implied capacity for offsetting carbon. That's what it costs now, but as more people participate, the cost will undoubtedly go up.
I don't think it's that straightforward. Stuff like planting trees should have the cost/ton go down over time due to scale (there's a land question but there's a _lot_ of federally owned land). A lot of other stuff could work similarly.
Mass production is a thing that brings costs down, so some of the carbon offset programs could take advantage of similar effects.
You're definitely right that some of these projects aren't infinitely scalable though. There are only so many gas stoves used in northern Uganda to be replaced..
For example, you could "offset" the effect of buying a daily latte on your household budget by just continuing to buy them while your partner promises to skip their own daily latte. Despite the offset, your bank balance would be lower than if you had just stopped buying them.
There is no known way to remove carbon at a scale that would make a difference and a cost people would accept. As almost all carbon from extracted fossil fuels is now in the atmosphere and ocean, it would amount to running our fossil fuel extraction in reverse.