I know this is obvious, but it's important to look at your online agreement or terms of service for customers to see what they've agreed to.

If you're not legally culpable, I don't see the reason for the refund. The fact is most people, especially in the US, will understand that delays caused by the government's servers are not at all unusual, and they might just side with you. Perhaps you're concerned about social dislikes before you've even gotten your startup off the ground. I find communication is really the most important thing; if there are few enough customers to contact, I'd call every one of them; it may see like a waste of time, but they're really key to your business, and having the founder contact them may bring tears to their eyes.

We are dealing with human beings, and so-called disasters could be turned around to super-power you. Remember that Intel's biggest growth was when there was a rare problem with their pentium chip, and they fixed and returned every one, and made a huge marketing campaign around it; if you have already refunded, you should make sure to let everyone know about it, and reach out to customers, perhaps via email directly. Testimonials are a really big deal in the social world, and every one of them could be your biggest advocate for lots of new customers.

Regarding what you're apparently most worried about, the bigshot late stage startup, that wouldn't be a concern to me. Some bigshot, who can't afford an EIN number; rmember what YC awards on here, it's called karma, and it goes around. If they try to injure you, they may have a lot more to worry about when you tell your side of the story, especially if they are bigger.

Let me tell you a true story that happened to me just today. I had an investor on my podcast some weeks ago. This person seems really nice, and has a huge social following, so I offered something that really goes against the way we operate, to do some free legal work for this person, so they could view our work, and if they were satisfied, to establish a regular working relationship together.

A few weeks later, this person reached out to take me on my suggestion (and was actually mad that I hadn't contacted the person earlier, but apparently the email had ended up in spam) and not only did we do the work namely fixing a really bad contract bringing a ray of sunshine to it (sorry for the braggadocio), and they kept all the changes, but I did a full video of explanation for this person and their partner.

Yesterday, they reached out to me again, this time stating that we could work together but that their partner is cheap, so watch the fees, then telling me how much we could charge, and when we mentioned our customary retainer for this sort of work, refused but did mention they needed the work done right away.

My response was to say sorry, we can't do it, and send them a polite email, and fortunately as founder like you I don't have to answer for it. I'm just letting you know about own experience, acquired over decades.

This is my hugely wordy way of saying to stay away from time wasters, big-shots or otherwise, and put your time into what counts. I've seen lots of folks like this, and all they do is waste your time, because you've got finite time, and let it be spent with the good customers, who love you and want to work with you. The 'big shot's' power is mostly a perception, and most people are fair minded. Don't give into it.

My only caveat is that I don't know the exact circumstances and I'm taking your word for it.