Hi,
That is a feature provided by HikaSerial and his plugins "attach serial" and "mail to custom field".
But in order to use "item custom field" or "order custom field" (to let the customer specify the email address), it also require HikaShop Business.
More about HikaSerial :
www.hikashop.com/support/documentation/2...serial-tutorial.html
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.