they’re not paying premiums. there is no “insurance policy” to pay premiums. when a company self insures itself, what that means, is, they keep some capital on hand (or readily availible,) so that they can weather a problem.
because they price the loss into the merchandise they sell, if they expect x% of the pallet to be stolen, and the reality is a bit higher, they dip into that fund to buy the next pallet, which, they then price at y% loss, and a bit more to compensate for the extra they lost on the first pallet. Maybe this time it was a bit low. so they go back to x% on the third.
the costs are passed directly onto consumers with no insurance company meddling. because that would just be inefficient. they might have a clause in a policy against mass-loss if, for example, the entire store gets looted in a mass-theft or if the store somehow goes up in smoke or hit with a hurricane. but as a matter of normal operations, they’re not claiming insurance on every bit of lost product regardless the reason.