Service products require time, and no one has any time. And there is a lot of spinning of wheels before you can ask the customer to pay. You may have to make a house call, and an earlier date is better than one next month. In this case the immediate objective is to get the appointment. Then it is down to you, eyeball to eyeball, to clinch the sale.
Solid products have to be delivered.
If goods are to be delivered then the target is 48 hours. If you promise 'within 28 days' again the customer may well go to a rival Internet website, or walk down to the shops. Instant-fulfillment means that you will have to sort out your supply chain. What is the fastest way to get the product to the customer. Maybe the manufacturer or wholesaler, way up the supply chain should dispatch the goods direct.