Haha, you’re asking at a good time since my head’s still buzzing with store research. This is all off the top of my head, so be sure to do your own digging too:
Etsy is a good option. Not only will it give your merch some good exposure, it automatically handles shipping labels, so all you have to do is print them. It’s not a manufacturer like Redbubble (companies like this aren’t supposed to be used for Undertale anyway), so you’d have to find your own third party companies to create your products. This is assuming you’re not doing it all by hand. :)
Etsy does take a percentage of your sale income, so just be sure to calculate that when you make your pricing. They also recently incorporated something that gives items with free shipping priority in search results (which is dumb). So if you do go with them, be sure to figure out a way around that, even if that’s just including shipping in your base price with options for domestic/international.
You can also use store hosting sites like Shopify, Storenvy, or Big Cartel.
Storenvy and Shopify are pretty similar. They take a percentage of your sale earnings for their services, which can be nice if you’re not expecting a lot of turnaround or slower traffic over a long period of time. They have add-ons that handle shipping and such, but I’m not sure how much they cost (if anything). I believe Shopify and Storenvy have a marketplace that gives your work exposure, somewhat like Etsy.
BigCartel, on the other hand, is a paid service. It’s completely free up to 5 products, but if you’d like more, they have different tiers you can choose from. Up to 25 products is only $9.99/mo. This one works for me because $10 is likely less than I would spend using a percentage system, and my shop isn’t even going to be open for more than a month. When it’s over, I can downgrade back to the 5 product tier and use that when selling leftovers. Easy. They also have shipping apps in the same way Storenvy and Shopify do, some of which come with free trials and no commitment. However, you will have to do your own advertising and rely on your following to help spread the word.
Well… that was a lot. I hope it helps! :)