According to Google Shopping policies, Unsecured checkout or a missing refund policy would cause the disapproval of individual items or an entire Merchant Center account.

Explanation: Google’s goal is to provide the best user experience on Google Shopping. In order to ensure that the ads Google shows to online shoppers are safe, secure, and comply with legal requirements, Google maintains comprehensive enforcement of the Google Shopping policies for all merchants.

Read more here: https://support.google.com/adwords/answer/6103800