Inline Entity Form

Another tip from the commerce guys on using inline entity forms to cater for product variations:

The Inline Entity Form provides a widget for reference fields that allows inline management (add / edit / delete) of referenced entities. It provides an excellent solution to the product display -> product UI problems, allowing the products (called "variations" in the UI) to be managed from the product display add / edit page. The module also provides a more modern line item management widget, that allows store owners to edit custom fields that are attached to those line items.