A purchase order is the record of what you are actually paying this time. Unit costs start from your Shopify product costs, and the Cost summary takes discounts, shipping and other charges that feed your landed cost when the goods arrive.
Costs, discounts and charges
Unit cost pre-fills from your Shopify product cost, and you can override it per line - the PO is the source of truth for what you're actually paying this time.
Discount % applies per line, for supplier deals and volume pricing.
Setting the same figure on lots of lines? Two shortcuts. When several lines are variants of one product - a size range, a colourway - typing the cost or the discount on one of them puts Apply to the other variants under the field, which copies it to the rest of that product's lines. The link only appears while they still disagree, so it doubles as a check that a range matches. For lines that are not variants of each other - a whole order at 15% off - tick the ones you want in the Select column, then use Set cost or Set discount in the bar that appears above the items. Your selection stays put afterwards, so you can set a cost and a discount on the same rows.
In the Cost summary, click Add cost and choose Add shipping or Add adjustment to add order-level charges - freight, handling, a credit from the supplier. Rename the labels to match the invoice, and choose how each charge splits across items. Those charges feed your true landed cost when you receive - see Landed cost: what your stock really costs.
For a discount on the whole order, choose Add discount. Enter it as an amount or as a percent of the subtotal - a percent converts to a fixed amount the moment you add it, so it will not change if you edit the items afterwards. The discount prints on the purchase order your supplier receives and reduces the order total. With the default By value split it also lowers each item's landed cost in proportion; pick Not allocated if it should only reduce the order total. Typing a negative amount on an adjustment does the same thing.
Good to know
A cost per unit can be a very small number - buy resin at $5.31 a kilo, stock it in grams, and each gram costs $0.00531. Stockroom keeps unit costs to eight decimal places everywhere it uses them: recipes, landed cost, averages and margins. Shopify's own cost box only holds cents, so the figure sent there is rounded to the cent, and a cost that would round to nothing is sent as $0.01 - a zero in that box reads as no cost at all. The exact figure stays in Stockroom, and order totals are money and unaffected.
The costs on a placed order are fixed; what a delivery actually cost is recorded when you receive, and can be corrected afterwards - see Receiving Inventory Against a Purchase Order.
How charges spread across items and reach your product costs is in Landed cost: what your stock really costs; buying in another currency is in Buying in your supplier's currency.