Unpurchased Items you have added to your cart should be populated from previous shopping every time you log in, regardless of whether you are on the same device or switch to another device for 14 days.
In the event that they do not populate when logged in, you can add them back using Account Settings.
Go into Account Settings (on top navigation bar)
Click on Retrieve Unsubmitted Cart with items.
If you have trouble with items being lost in cart and you are unable to have them re-populate, please contact support at firstname.lastname@example.org