When does possession pass from seller to buyer when an item is shipped?a) upon drop-off at door step.or b) upon buyer picking up from doorstep.

The answer depends on what the contract provides.  If the contract is silent, it sepends on whether the seller is a merchant (in which case risk of loss passes when the buyer takes physical possession of the goods) or not (in which case risk of loss passes when the seller tenders the goods to the buyer, which would certainly not be later than when the goods are delievered to the buyer's doorstep).