The ornamental design for a soap-holding shower sack, as shown and described.
FIG. 1 is a front elevational view of a soap-holding shower sack showing my new design;
FIG. 2 is a rear elevational view thereof;
FIG. 3 is a left side elevational view thereof;
FIG. 4 is a right side elevational thereof;
FIG. 5 is a top plan view thereof; and,
FIG. 6 is a bottom plan view thereof.
The indicia or broken lines shown on the figures are for illustrative purposes only and are hereby disclaimed as forming no part of Applicant's claimed invention.