The ornamental design for a petal for a shotcup, as shown and described.
FIG. 1 is a perspective view of the petal for a shot cup from the proximal end of the shot cup;
FIG. 2 is a perspective view of the petal for a shot cup from the distal end of the shot cup; and,
FIG. 3 is a side elevation of the petal for a shot cup.
The broken, dashed lines in the figures are included for the purpose of illustrating environment of the claimed design and do not form any part of the claimed design.