When may the seller demand payment in an installment contract?

Generally speaking, the UCC assumes that a contract for goods must be delivered as a whole (in a single lot) unless the contract says that the goods can be delivered in multiple lots. Contracts that can be delivered in multiple lots are called "installment contracts." A seller can demand to be paid for each lot that they deliver if its possible for them to adequately apportion the price. In other words, if someone orders 1,000 identical widgets for $1,000, and those widgets are sent in 10 different shipments, then each lot of widgets would have a quantity of 100. Since they are all the same widget, you can easily apportion the price of each lot: $1,000 divided by 1,000 widgets is $1 per widget. 100 widgets per lot is $100 per lot.