What is the difference between worship and other acts, and why did scholars divide deeds into acts of worship and transactions in terms of stipulating sincerity for them, even though the entire life of a believer is an act of worship? And what are the interests that the Lawgiver permitted the مكلف (accountable person) to intend, which do not fall under showing off (riya')?

Pure acts of worship differ from pure customs. Acts of worship are legislated for the glorification of Allah and seeking the reward of the Hereafter, and sincerity therein is a condition, while showing off (riya') is forbidden. As for customs, they are established for worldly interests, and sincerity or intention is not a condition for them, nor is one rewarded for them unless one intends by them to draw closer to Allah and seek the reward of the Hereafter, which then brings them into the concept of worship, just as a custom intended as an act of drawing closer becomes worship.

Al-'Izz ibn 'Abd al-Salam said: "That which is legislated for worldly interests and to which otherworldly interests are only secondary... one is not rewarded for it if it is intended [for its worldly benefit] unless one intends by it closeness to Allah - Exalted be He."

Al-Shatibi said in "Al-Muwafaqat" that working to rectify current customs, such as marriage, buying, and renting, does not contradict the intent of the Legislator. If seeking worldly gain in them were a flaw, they would be equal to acts of worship in requiring intention, but customs do not depend on intention, so showing off in them does not invalidate them, unlike acts of worship whose purpose is the glorification of Allah.

As for how to achieve this, it requires religious knowledge to understand the interests that the Sharia permits intending in acts of worship, whether from the Quran, like trading during Hajj, or from the Sunnah, like preserving one's chastity through fasting.