Authorize.net payment plugin showing error while purchasing product in Hikashop

Hi,

Well, the FedEx error message indicates that the address entered by the customer is missing the postal code or that it is invalid. So you need to make sure that the "address_post_code" custom field in the menu Display>Custom fields is published and required.

Regarding the transaction declined error, the error message actually comes from Authorize.net.
There are many reasons for why a transaction can be refused by Authorize.net
Here is a documentation on the issue:
support.authorize.net/s/article/Why-was-my-transaction-declined
So I would first recommend to check what is written about this transaction in your merchant account. You might leanr more about why that problem occurred.
I would also recommend to inquire to Authorize.net merchant support about this transaction if the information from the merchant account isn't enough to help you understand the situation.