The ornamental design for the flip-top sealing cap, as shown and described.
FIG. 1 is a left side view of the closed flip-top sealing cap; and
FIG. 2 is a right side view of the closed flip-top sealing cap; and
FIG. 3 is a back view of the closed flip-top sealing cap; and
FIG. 4 is a front view of the closed flip-top sealing cap; and
FIG. 5 is a top view of the closed flip-top sealing cap; and
FIG. 6 is a perspective view of the closed flip-top sealing cap; and,
FIG. 7 is a perspective view of the opened flip-top sealing cap.
The dashed lines which indicate an object being sealed by the flip-top sealing cap are included for the purpose of illustrating environmental structure and form no part of the claimed design.