Is it obligatory to rely on Allah in all matters, even in simple things like opening a door or buying an item, and must one believe that outward actions are merely means? And what is the difference between a sin, minor shirk, and major shirk concerning theft?

Reliance upon Allah is obligatory, and it is the heart's dependence on Allah. It is not complete except by taking up means (performing actions). Therefore, taking up means is an act of obedience to Allah, and it is from the actions of the limbs, whereas reliance is an action of the heart. Taking up means does not contradict reliance; rather, the reality of Tawhid (monotheism) is not complete except by taking up means. As for attachment to other than Allah, it is of categories: what contradicts the foundation of Tawhid, which is major shirk (polytheism); what contradicts the perfection of Tawhid, which is a type of shirk; and what does not contradict Tawhid, which is the attachment to the means merely as a cause, while the primary dependence of the heart remains on Allah.