How to implement a Drupal Commerce multi-select membership donation form?
I know you are asking about the form specifically, but it isn't clear to me how you are actually managing your 'memberships'. Something like CiviCRM that has full membership management tools can be integrated with both Drupal and with Commerce. There is a module for the latter, but we have found it problematic and have instead done the Commerce to CiviCRM integration using Drupal Rules and the CiviCRM Entities modules.