No one is going to give you a medal for choosing to have less comfortable testicles so that you can be ever so marginally richer.
It's simple. Do the math. Calculate how much sooner you could retire if you bought cheaper underwear, and then ask yourself if working that length of time if worth the sensation of cozy genitals.
Also, just FYI, there are plenty of brands of soft, microfiber underwear that are a lot cheaper than $30/pair. But if there's a reason you want to support this particular company, them again, run the math and see if it feels worthwhile.
The bigger issue here is the guilt over spending. Guilt over spending is irrational. These are decisions about your quality of life. They impact you. So feeling guilty about them makes no sense because you are the one who is in charge of determining what is best for you.
It sounds like you struggle to judge what is intuitively best for your overall quality of life though, and that is an issue. Because you have to be the one who makes ALL of the determinations of what is worthwhile to invest in, whether that investment be money, time, energy, or attention.
The only way to live well is to deeply understand the best uses of all of these resources for optimizing your overall well being.
Remember that money isn't anything in and of itself, it's just a placeholder for time and energy resources, whose value change over time. So it's up to you to understand what exchanges of resources are most valuable.
Is comfy underwear worth the time/energy exchange they cost? It's a yes or no question, no guilt involved.
If they are clearly worth it, then why would you feel guilty about them? Do you feel guilty about other things that are clearly worth it? Like, do you feel guilty buying food that tastes better than cheaper options that meet your nutritional needs?
Where is the line where something unnecessary is worth the it?
I applaud you for questioning if the underwear are worth it, but not for feeling guilty over a decision that impacts you and your quality of life. So what if you choose to work longer for underwear?? Who cares?? Why would you feel guilty about that? Who are you harming??
And what purpose does the guilt even serve??? Who is it for??? Who is being harmed by you deciding that any given thing is best for your quality of life??
Sure, if something could be found more ethically be going cheaper or used, then some guilt for spending for luxury might make sense. Or if you could just go without and you have a strong anti-consumerist value system, but you can't go without.
A quick look at the company website shows a huge amount of marketing around ethics and sustainability, so it's arguably that you should actually feel more guilty about not buying their underwear just to hoard more wealth for yourself. My DH's underwear is cheaper and probably made in Bangladesh. And somehow you should feel guilty for your purchase from an ethical company and he shouldn't feel guilty about his???
Make it make sense!
There is zero reason to feel guilty over this, so I encourage you to reflect on what the fuck the purpose of the guilt is and where it comes from.
Examining purchases critically is absolutely of vital importance for determining how to optimize your use of resources, but if something is a good value trade, why the fuck would guilt come into play??