Had postponing those days been sinful, ʿĀʾisha—may Allah be pleased with her—would not have been allowed to persist in it.
That without which a command cannot be carried out
When the performance of a commanded act depends on something else, that prerequisite is itself commanded. If the original act is obligatory, the prerequisite becomes obligatory; if the original act is recommended, the prerequisite is likewise recommended.
Obligatory example: covering the ʿawra (private parts). If this cannot be done without buying a garment, purchasing the garment becomes obligatory.
Recommended example: wearing perfume for Jumuʿah (the Friday congregational prayer). If that requires buying fragrance, then acquiring the perfume is recommended.
This rule is part of a broader maxim: the means take on the rulings of their ends. Thus, the means to obligatory acts are themselves commanded, and the means to prohibited acts are themselves forbidden.