Until and unless you've actually been sued in a relevant court, you don't need to do anything (and frankly probably shouldn't, it's like replying to spam). It appears Spark Fun has actually been sued in this instance, which is a slightly more aggressive form of trolling. IANAL but I am a very experienced serial entrepreneur (and after a few decades you've pretty much seen all the relevant flavors of this shit). Spark Fun probably didn't say anything about their specific response in this case because the general and prudent advice is to never publicly discuss anything about an active suit without really good reason.

Since the initial demand from the plaintiff already names a price for settlement, that's a huge sign they aren't serious. The fact it's $500 further reinforces this is a routine bulk scam. In theory, if you don't respond at all they could get a default judgement when you no-show at the eventual hearing date. The odds are overwhelming they don't ever show up for these, plus they'll (usually) also need to write and pay to file some actual, specific argument in advance of the hearing which you'll be able to see (when and if they actually filed anything) putting you on notice they are actually playing.

If you're a cautious person and you or your firm have significant enough business interests to care at all, then the prudent thing to do is send it to your lawyer and tell them you authorize 15m billing to read it and 15m to draft a response, basically saying "fuck off". That way you're fully covered from a legal standpoint. If you're just a guy in a garage selling some stuff but aren't incorporated and don't really make any money, then you could just ignore it. Once again, this is only because they filed a suit. Just a random letter from an obvious, known troll should be tossed. If you decide to respond, be careful your diligent lawyer doesn't run up >$500 just drafting up a routine response (which happens).

If you're a garage guy, you can probably find a pretty decent 'mad libs' suit response online you can adapt yourself to just file. It doesn't HAVE to come from a lawyer (although if it were a serious suit it's always a good idea to get a lawyer's input). At $500, the troll will lose money if they respond to you at all (they already had to pay some minimum filing and service fees).

I've had countless dozens of these patent troll registered letters and zero of them ever amounted to anything at all. In all these years and across dozens of products and technologies I've had exactly ONE sort-of serious patent claim. It was easily recognizable because it was a "letter of concern" from a long-time leading company in that product space. They've been around decades and have lots of patents. In some ways, they pretty much invented the whole area. So I took that pretty seriously. I talked about it with a good patent lawyer and also read the patent and it's clear that our software wasn't actually doing what their hardware did 15 years ago. Our approach worked great but it was a bit of unorthodox counter-intuitive cleverness (aka hack). We called them up and determined they didn't really know "how" our software worked and were just assuming it might infringe their patent. We told them it didn't but they were still kind of concerned, so our lawyer suggested "Hey, they seem pretty reasonable and they aren't that much bigger than you, why don't you just fly down there and white board just enough of your approach that they understand it's very different."

So we flew down and met with their CEO and CTO and explained until their CTO said "Oh, cooool." They went and talked for a few minutes, came back, thanked us for coming down and gave us a really nice tour before we left. Yes, we possibly disclosed a small part of a trade secret but the reality was those guys were never going to compete with us as we were going after different markets. Also, they clearly had nowhere near the low-level software chops to emulate what our code did. At the end of the day, the typical lawyer approach could have put us through discovery and perhaps a summary judgement (and a hundred grand in costs (min)) but looking at the circumstances and just being reasonable was the smart play. In all my years in business I've never been to court over a patent thing and only had to go to court once because, early on, I did a deal with a crazy sue-happy partner who I didn't do enough due diligence on. Lesson learned - and in that case, the litigious idiot ended up dropping his suit when time came for discovery and then paying all our costs just to get out of the counter-suit we had to file. So... worked out okay but it was expensive, distracting and annoying for six months.