Hi,
If it is an address custom field, you can read the value in:
$data->cart->billing_address->my_custom_field
OR
$data->cart->shipping_address->my_custom_field
If it is an user custom field, you can read the value in:
$data->customer->my_custom_field
Just replace "my_custom_field" by the custom field key you're using.
Regards,
Jerome - Obsidev.com
HikaMarket & HikaSerial developer / HikaShop core dev team.
Also helping the HikaShop support team when having some time or couldn't sleep.
By the way, do not send me private message, use the "contact us" form instead.