I just did this for the first time and I got an email which began:
So you've ordered a giffgaff SIM. Nice work. Your order number is *****.
You'll receive another email when your SIM has been dispatched to let you know it is on its way to you. Typically it takes around 3 working days from now until the SIM pack lands on your doorstep - if your order was placed before 4pm and you have a fast postie you might even get it tomorrow!
Once you receive your SIM you will need to head online to giffgaff.com to activate it - check out the step by step guide here.
But of course this SIM isn't for me (and indeed my friend, not me, has already had the "SIM is despatched" email) so the wording of this email really needs tweaking to be clearer that it's acknowledging an order for a friend not for you.