I have a Mavik ordered but not yet shipped. Is it possible to add DJI Refresh to the order now. (I obviously don't have a serial number yet)

Just order it once you receive it. You have 48 hours from when you activate your mavic.

Just as FrequentFlyer mentioned. DJI Care Refresh can be purchased for your Mavic that has not been activated or has been activated for less than 48 hours.

Dont purchase it before you receive it, as you will lose numerous days off the plan, as they activate it the day after shipping, meaning its activated 2-3 days before you even take delivery of the Mavic. Better to wait until you are going to use it, then buy and activate the Refresh care pack.

Thanks for the advice. I'll wait until it arrives.