Mrs Roper, whose family are sixth generation showmen, said: "We were delighted to receive it.
"Obviously the royals have got connections with the fairgrounds with the royal charters and it's nice that they support their local fair, the King's Lynn Mart."
The Mart, which pitches up on the Tuesday Market Place each February, was granted a Royal Charter by Henry VIII in 1537.
The fair, which traditionally opens on Valentine's Day, marks the start of the travelling showman's year.
West Norfolk council asked Mart operators for their safety proposals for a Covid-secure fair in October. But since then, Norfolk has been placed in a Tier 4 lockdown.