"My eyes bulged out of my head," Sheldon told ESPN.
The card was graded a 9 out of a possible 10 on Saturday by Professional Sports Authenticator, a card and memorabilia grading service.
It is only the eighth 1955 Bowman card of Mantle ever graded 9 by PSA, and the first in more than 20 years. Only three have ever been rated a 10 by the grading service, Sports Collectors Daily reported.
Rothe, who was working as a third-generation bookbinder in Maryland, decided to check the video of the pack opening when he discovered he had won the Mantle card.
"My friend told me my face went pale white when I saw it," Rothe told ESPN. "I was weak in the knees."
Sheldon said he already heard from a collector who offered $50,000 for Rothe's card, ESPN reported.
Rothe told ESPN he was going the sell the card, but did not say whether he had accepted the $50,000 offer, Sports Collectors Daily reported.
"I have the card in a (PSA graded) 3," Rothe told ESPN. "That's good enough. I'll use the money to get a (Roberto) Clemente rookie and maybe get a lower-graded Mantle rookie."
Bowman, a card company based out of Philadelphia, was the main player in the baseball card industry until it was bought out by Topps in 1956. The 1955 set was the company’s final baseball card set until it was revived by Topps in 1989.