Provides Drupal Commerce with the ability to sell registrations via the Entity Registrations module.
This module, is still in beta. That means there can be substantial code changes, although the schema is relatively stable. However, this does not completely remove the possibility of any major changes that will affect interaction between this and entity registration. Check the issue queue for any possibly existing problems.
Use Drupal Commerce as usual.
If you don't already have a product display set up, create one with a product reference field.
Add a Registration field to the commerce product type you want.
Create a product and set the Registration type (or create one if you haven't already), add a new product display with that reference, and add the settings to the DB as above.
Add the product to your cart, go through checkout process and after the User Information checkout page/pane, you get to the Registration Information page/pane. This pane has the registration fields in a fieldset according to the registration type you set for the product. It will also have multiple entries depending on the quantity of the line item (purchasing two registrations? it has 2 sets of fields).
Once you get to review, there should also be a Registration Information pane with the entered info for each registrant.