Don't just eat the loss and move on - this is worth pushing back on, especially since it wasn't disclosed upfront. I've been on the receiving end of this myself. A few years back I ordered some workout equipment from a smaller retailer and they tried to hit me with a 20% restocking fee on something I never opened. When I checked their return policy page, the fee was buried in tiny text at the very bottom in a section I'd somehow missed during checkout. I called them out on it, pointed out that most people wouldn't have seen that, and they actually waived the fee. The company basically admitted they should've made it more prominent.
Here's what I'd do first: go back through their website and look for where they disclose this fee - their return policy, FAQ, checkout pages, etc. Screenshot or document everything. Then contact their customer service and explain that you couldn't find clear disclosure of this fee before you made the purchase. Be straightforward about it, not angry. Most companies will work with you if you can show the fee wasn't obviously presented upfront. If they dig in and refuse, you've got options - you can dispute the charge with your credit card company and explain that a material fee was hidden from you at purchase.
The reality is that while retailers *can* charge restocking fees, the disclosure has to actually be reasonable and visible. "Hidden in a 9-point font on page 47 of the terms" doesn't cut it. You've got leverage here, so use it politely but firmly.