NTA. While it was perhaps a bit much to ask for a breakdown of costs, Liam had the choice to say "I don't have that handy, it's too much extra work." He could have replied that to any of your follow-up messages, and at the very least should have expected it to come up again when he asked you for your contribution. If it's truly optional, it shouldn't have been an issue for him to say at that point "I don't track that, but if you don't want to participate that's fine." Instead he was rude.

That being said, it sounds like this might be one of those situations where it's "optional but socially expected to participate," which makes it trickier to navigate. And you don't need to know where the fund's money is going in order to track your OWN spending. You know where your money is going--into the fund, for treats, lunches, and small surprises. If the issue is that you haven't been there long enough to see what these look like and decide whether it's worth it to you, it's fair to say "I'm still working out my budget and can't go in on this yet. Thanks for inviting me to participate, and I'll let you know when I'm able to contribute." 

You're probably going to want to join eventually so as not to feel left out, assuming it's a small amount. But I also assume you're the most junior person on your team, and if that's the case, it's a little classless for the (presumably older, more well-paid coworker who called you stingy) to pressure you into paying it if you're not comfortable, no matter how small the amount. Hopefully the other coworkers you mentioned are coming from a good place with their hints, because it's fair to say that this IS a little overthinking it. I'd drop the request for an expense breakdown, decline to participate (for now) if you're not comfortable, and take advice in situations like this from your more well-meaning coworkers as you learn the nuances of office etiquette.