This isn't good. If someone has taken the effort to calculate up exactly how much they owe after tax and tip, you should NOT demand more than they owe. If they want to donate to Toady One, then they should. That is the whole principle behind the donation method of support. You don't force it. It is all given freely. This shouldn't change because we are having a fan meetup. The prices there are already very pricy for where I am from. Some people may be coming from places with even lower cost of living. If people need to worry about their cash that much, indulge them. if you are incapable of doing this, I will be perfectly willing to take the duty from you.
I'm sure your intentions are good here, but you seem to be going out of your way to misunderstand. TL is not trying to extort donations from people, he's making a commendable and proactive attempt to avoid the
inevitable mistakes that will arise when everybody's handing him stacks of pennies, and he's forced to hope that the amount's right because he doesn't have time to count up 30 people's change, and then forced to pay out of his own pocket at the register to make up for the several dozen dropped coins. The fact that the extra is going to B12 is an afterthought.
If you're trying to offer help, do so without the self-righteousness. All you needed to say was, "If counting out the change is too much trouble, I can take care of that for everyone and make up any differences myself." Although you still shouldn't expect him to accept your help, because you are after all a stranger offering to take charge of a large amount of money, and you don't have the credibility of having organized the previous event.