The module provides the new field type "Line item type reference" with this field type you can define which additional line item types belong to a module.
When adding a product to the cart the referenced line item types are taken in account and if a referenced line item type exposes field to the end-users the field can be edited by them.

Shipped with support for ctools content type "entity field". So you can use it with Panels / Panelizer.

1. Create line item types containing the fields you'd like to provide for product customization.
2. Place a field of the type "Line item type reference" onto the product display node you want to make customizable.
3. Configure the "Add to cart" on your product display node display settings to handle a "Extra line item types field".
4. Edit the product display node and select the line item types you'd like to attach to this product when adding it into the cart.
5. Profit!