This is one of the oldest Willshire postmarks that I have, if not the oldest.
The envelope is postmarked Wilshire, O, Mar 11.
The year is not included in the postmark but the letter inside the envelope is dated March 10, 1879, a Monday evening. It was postmarked the next day, the 11th. Just to double-check, I looked at a March 1879 calendar and 10 March 1879 was a Monday.
The letter is addressed to Mr. Isac Funderburg, New Kerlisle, Ohio, Clark Co.
This town is actually spelled New Carlisle and is near the city of Springfield. I am more familiar with Ohio’s other New Carlisle, in Walnut Creek, Holmes County. Since they did not have zip codes in 1879 it was probably a good thing that the sender also wrote Clark Co. on the envelope.
Below is a transcription of the letter, as it was written, with the spelling as written, but I added some punctuation where I thought he may have intended, placed in [ ].
March 10, 1879
Mr. Isaac Funderburg
I have got bought 8 hed of cattle [.] I think I can fetch for 20 hed besids yours but I may not get to your house till Monday or Tuesday next week [.] I will bring your mare a long with me. The man you bought her of will ceep her till I come down. I will be there as soon as I can get there and do the best I can but if I can’t get there till Monday or Tuesday I will rite to you when I will come.
Yours as ever
D B Mills
I may get there by Sunday if I can.
I wonder how they transported livestock back then.
The intended recipient of the letter, Isac Funderburg, was born in Ohio about 1840 and in 1880 lived in Bethel, Clark County, Ohio. He was a farmer, was married to Ester, and had 7 children. 
According to his obituary, D.B. Mills (1842-1887), the letter-writer, lived on a farm in Willshire Township. He died 20 April 1887, after suffering 8 months with scrofula. Scrofula is lymphadenopathy of the neck, also known as tuberculous of the throat, usually caused by an infection in the lymph nodes, and sometimes caused by the tuberculous bacteria. He was 45 years old and left a wife and 3 children.  D.B. Mills is buried in Willshire Cemetery and served in the Civil War. 
 1880 U.S. Census, Bethel, Clark, Ohio, ED 34, p.4D, dwelling & family 74, Isac Funderburg; Ancestry.com, viewed 22 Jun 2023.
 D.B. Mills obituary, 29 April 1887, Van Wert Weekly Bulletin, Van Wert, Ohio; NewspaperArchive.com, viewed 22 Jun 2023.
 D.B. Mills, Find a Grave.com, memorial no.96096712, 1842-20 Apr 1887, Willshire Cemetery.