Even if you choose to let it go, I would still tell the artist that the pose was wrong. Something like "I'm happy with what I got but when I said 'presenting' I meant a butt-view." That way the artist will know for next time someone asks for that pose assuming there was a language barrier and they didn't just forget. They might offer to fix it for you anyway.

If it were me and I liked the art, I'd probably offer to pay for a new commission (this time in the pose you -actually- wanted) and keep both versions.

Another option besides asking them to redo it would be to ask for a partial refund, though a partial refund of $5 might not be worth the hassle.