Is This An Unusual Usage Or A Valid Postal Rate?

I'm just a bit confused as to why the stamps total 21 cents?

The #17 indicates it may be a registered letter. Registered mail delivery began in 1855.
Regular postage at that time was 6¢ for the first ounce (weight). Perhaps the 15¢ was for registration.

It says on the bottom upside down it says "$46.00 was enclosed"
So it was registered for that purpose?

Possibly. Don't know much about the US history on registered mail (bigger brains here can fill you in) but in Canada registered mail started out as Money Letters, which were exactly what they sound like. You would pay an extra fee to ensure money enclosed in the letter was tracked and made sure it got to it's destination. They actually wrote Money Letter on the front of the letter to indicate what it was.

The Canadian Registered mail system was also introduced in April of 1855. Handstamps which read REGISTERED replaced MONEY LETTER on the front of the letter.

Hastings is a river town on the Mississippi and Hokah is very close to the river, any chance this went up the river on a boat?

How would I find out stallzer?
Also before anyone asks....nothing on the back!

Registry fee is 15 cents at this time.
Postage is 3 cents per half ounce at that time, thus a double rate cover.

Both $46 references were added after receipt as docketing.

I've never had a #163 on a cover before!
Thanks for your knowledge John

You still don't have a Scott 163. As the date of usage is 1871 the stamp should be a Scott 152 as Scott 163 was not issued until 1873. There is a slim, but possible chance your 15c stamp is grilled. You should check the 3c stamps for grills as well.