Hi Amala,
as we just had a quick chat about, this is only possible on the checkout screen - Under the "My Selections" header there's the option "Save as Request and submit at another time." This saves the request in a "Not Submitted" state so you can go back and complete/add to it later.
However, it does have two limitations, due to being on the checkout screen:
1) You can't use it with a 'Cartless' service, as you can't get these into the cart, by definition.
2) You can't get to the cart checkout until your choices have been validated, so any mandatory fields or with other validation requirements do have to be completed.
If either of these limitations are an issue, you'd need to raise an Idea:
thanks,
Iain