The Bell voucher is a new redeemable item in New Horizons. They can be redeemed from the Nook Stop in the Resident Services Tent or Building. They cost 500 Nook Miles, and are sold for 3,000 Bells. They have no uses other than as a way to convert Nook Miles into Bells.
Bell vouchers are more efficient in converting miles to Bells, as Nook Miles tickets sell for 10,000 Bells each; however, because both tickets stack to a maximum of 10, a stack of Nook Miles tickets would be worth more than a stack of Bell vouchers.
During the May Day event, 9 Bell vouchers can be obtained by completing the maze using as little fruit as possible.