They will want to sell either 120 or 190 (depending on the level of demand) but the most they can sell is however many fit in the van
If they have the small van, then it will only hold 100 units and so they can only sell 100 units and so the sales are 1,000 (whatever the level of demand).
If they have the medium van, then it will hold 150 units. So if demand is 120 they will sell 120 and the sales are 1,200. However if the demand is 190, they can only sell the 150 that fit in the van and therefore the sales revenue will be 1,500.
If they have the large van, then it will hold 200. So if demand is 120 they will sell 120 and the revenue will be 1,200. If demand is 190 then they will sell 190 and the revenue will be 1,900.