We're running ZenCart version 1.5.6c
It does have a good bit of customization done to it, and 3rd party modules installed.
For our ZenCart checkout, we are using the One-Page Checkout module ([login to view URL])
Currently, a customer can make a particular address as "Primary," however, it will make it be primary for BOTH Billing & Shipping. We want the ability for our customers to have the Billing address different from the ship to address, but also to be able to specify a Primary Shipping Address that is used by default if multiple addresses exist in the address book.
Please state your experience with ZenCart - How many years and since which version have you been involved?, some sites you've developed,
How well do you know JQuery, CSS, PHP, MySQL?
What are the top 5 modules (3rd party) that you install?