The ornamental design for a sock for use with flip-flops, as shown described.
FIG. 1 is a perspective view of a sock for use with flip-flops while being worn on a foot with a flip-flop sandal showing my new design;
FIG. 2 is a perspective view thereof while lying flat;
FIG. 3 is a top plan view thereof;
FIG. 4 is a bottom plan view thereof;
FIG. 5 is a side view thereof; and,
FIG. 6 is an opposite side view thereof.
The broken line showing of a leg and sandal in FIG. 1 is included for the purpose of showing environmental portions of the sock for use with flip-flops and forms no part of the claimed design.