You'd have to make creation of an account mandatory. Adding to an order is OK, but the ability to change an order would HAVE to have an admin determined window of opportunity - else, you'd be inviting people to change the color of a shirt after it ships. If the store doesn't yet know the item has shipped from a drop-shipper, you'd have to call the shopper and then un-do the changes back to the original order. The original order must be retained for reference.

An altered order would have to kick off a new round of admin notices, drop-shipper notices, and create a new order number for accounting and QB / Shipper integration, crediting, cc processor tracking, etc.

This can get troublesome, but perhaps worthwile for stores offering off the shelf items and a bullet-proof inventory system if they are tracking inventory.

I'd pass on it - most of my items are built to order... I can't have an order change days after the order is placed.