“Beggar’s can’t be choosers” is an old proverb. A supermarket joke was printed in the Washington (DC) Post on February 29, 1996:
 
“Checkers at grocery stores always ask whether we want paper bags or plastic. Why can’t checkers make that decision themselves?
 
Lewis A. Rhodes, of Silver Spring, says it’s because baggers can’t be choosers.”
   
             
Wiktionary: beggars can’t be choosers
Proverb
beggars can’t be choosers
1. When in need of help, one cannot dictate how that help is given; when one’s situation requires one to beg, one cannot complain about the insufficient or substandard gifts that one receives.
 
29 February 1996, Washington (DC) Post, “Bob Levey’s Washington,” pg. S13, col. 2:     
Checkers at grocery stores always ask whether we want paper bags or plastic. Why can’t checkers make that decision themselves?
 
Lewis A. Rhodes, of Silver Spring, says it’s because baggers can’t be choosers.
 
Google Groups: alt.callahans
Overheard At Grocery Store
Osty
2/27/98
Bagboy, “Would you like paper or plastic?”
Customer,  “I don’t care, whatever.”
Checker, “There, who said baggers can’t be choosers.”
 
Google Groups: alt.callahans
“Puns of the Weak” 8/11/00
Stan Kegel
8/13/00
(...)
Grocery clerks make you pick paper or plastic because baggers can’t be choosers. (Jay Salinger)
     
19 October 2000, San Francisco (CA) Chronicle, “Musing for Anything Amusing” by Scott Ostler, pg. A2:
At Safeway, the young man bagging asks Arthur Ost the usual: “Paper or plastic.” Arthur says, “I don’t care, you decide.” And the kid says(Arthur swears), “I’m sorry sir, but baggers can’t be choosers.”
 
Google Groups: alt.humor.puns
Puns of the Weak 1-3-03
Stan Kegel
1/4/03
(...)
Grocery store clerks make you pick paper or plastic because baggers can’t be choosers. (Jay Salinger)
 
Google Groups: alt.humor.puns
Puns of the Weak 09-05-03
Stan Kegel
9/5/03
(...)
Grocery clerks make you pick paper or plastic ‘cause baggers can’t be choosers. (Renee from Napa)
   
Google Books
It’s Better to Die Laughing Than to Be Dead Serious:
How to Be the Life of the Party, the Podium and Everywhere in Between
By Marvin Maupin
Bloomington, IN: AuthorHouse
2010
Pg. 173:
Grocery store clerks make you pick paper or plastic because baggers can’t be choosers.”—Jay Salinger
 
Reddit—Jokes
Why do they ask you if you would like paper or plastic at the grocery store?
submitted November 5, 2012 by DelDelDel
Because baggers can’t be choosers,
   
Google Books
Curse of the PTA
By Laura Alden
New York, NY: Obsidian
2013
Pg. ?:
Question: Why did checkout lines start asking, “Paper or Plastic?” Answer: Because baggers can’t be choosers.
 
Reddit—Dad Jokes
Dad joke from my grocery store days. (self.dadjokes)
submitted March 14, 2015 by NoUrImmature
I worked at a grocery store for several years, sometimes as a bagger. Sometimes my interactions went like this:
Me: paper or plastic?
Customer: it doesn’t matter, you choose
Me: I’m sorry, I can’t do that
Customer: why not?
Me: because baggers can’t be choosers
(Groans)
 
Reddit—Jokes
Why do they ask you if you want paper or plastic at the supermarket?
submitted July 2, 2015 by thatnotalentassclown
Because baggers can’t be choosers.
   
Reddit—Jokes
“paper or plastic?”
submitted March 19, 2017 by danconnr
because baggers can’t be choosers
 
Reddit—Oneliners
Grocery store workers must let the customer decide if they want paper or plastic because baggers can’t be choosers.
submitted March 31, 2018 by wonliners