More about extra fields
You can find other examples of more complex functionality on this page: https://gist.github.com/makfruit/e410e1f0ab46656beda8df46c1666154
Advanced customizations to extra fiels are also available in the section above.
When saving information to an order, make sure your code follows these limits:
Total storage All extra fields for order must not exceed 8Kb of data in total. If an order has over 8Kb of data saved, Ecwid will only save the data not exceeding the limit.
Limit per field’s attribute The field’s value, title or other attribute must not exceed a limit of
255characters. If it exceeds the limit, the field will not be saved to order.