EE & CartThrob: How to pass custom field variables from Cart to Checkout pages

Question

In my CartThrob {update_cart_form}, I have a Matrix-populated zip code drop-down field. When a user clicks "Checkout" I want the value of the drop-down menu to pass as a variable to the checkout page and populate the billing zip code field. Is this possible?

EDIT: You can actually view the precursor to this by going here: http://dandelionflowershop.com/index.php/website/cart

Keep in mind that you need $40 in your cart before delivery options become available.

Answer 1

You should be able to save some custom data to session using the custom_data array like this:

<select name="custom_data[whatever]">...</select>

Docs: http://cartthrob.com/docs/tags/global_form_settings/index.html#form-fields

The value should then be available in the CT checkout tag like this: {custom_data:whatever}

Docs: http://cartthrob.com/docs/tags/global_tag_variables/index.html#custom_datayour_data_field_name