I should add - my store's permalinks are set up like so:
All shop pages and their sub pages are linking fine. However, when a customer finishes checkout the url they get (and this setting off a 404 page) is:
(obviously I made up those numbers, but that's the general idea).
If I take that incorrect URL, and add "shop/"just before "checkout", then hit enter, I can actually see that transaction's confirmation page just fine.
If I enter just sitename.com/cart, it automatically prepends "shop/" before cart and shows me the correct page. Same with sitename.com/checkout. It seems when it's time to show the checkout confirmation/Thank You page, the confirmation page's URL is not adjusting accordingly. It remains with the incorrect sitename.com/checkout/order-received/etcetcetcetc
For now I've set my 404 page to redirect to the homepage, but it would be most helpful to find a solution for why my shop permalinks aren't being followed at the very end of checkout.