The problem with the "pay them to fix the bugs" model is that it turns bugs into a source of revenue and creates the wrong incentive for the developer.
I'm much more inclined to agree to a "pay for additional features" model instead. Because then, by definition, you are paying for extra stuff, vs. fixing what should have been working correctly in the first place.
That's clever but wrong here: labor is what's being monetized. The people that they're asking probably require payment for their own labor and pay others w.r.t. other goods. Then, they OSS developers owe them free products and support of highest quality.
Quite a double standard. Requiring pay for fixes only incentivizes pay for labor, which is rare for OSS.
I'm much more inclined to agree to a "pay for additional features" model instead. Because then, by definition, you are paying for extra stuff, vs. fixing what should have been working correctly in the first place.