On a WordPress / CiviCRM installation, we need to do a bunch of customization on its payment pages. We want to do it in WordPress since there are WordPress plugins that can help do things like validate addresses and calculate sales taxes that are not yet available in CiviCRM, and the budget is constrained. The main integration we want is to write to CiviCRM 1) contact info, 2) invoice data for memberships, participants, and 'contribution' items like book purchases, and 3) payments received.
We're going to start by upgrading the CiviCRM API to allow multiple line items including Sales Taxes to be added properly.
Is there a good starting repo for the WordPress plugin anywhere? I've contacted Josh at http://www.joshcanhelp.com/wordpress-ecommerce-site-integrated-civicrm-migrated-drupal-6/#comment-30644. Alternatively, custom non-generalized implementations would also be fine.