Many Sites have prohibitive policies. Taking ownership of Slow shipping, bad delivery the product. Violations of the schema are potentially the most disturbing problem for ecommerce users. These violations go beyond just improperly handling some expectation, they jarringly confront the user with an unexpected and threatening event. Two unfortunately common violations are the requirement to register in order to use the commerce site, and any ambiguous error within the actual ordering flow.
Registration is a major sore point with commerce sites.
Users do not expect to authenticate themselves before they are ready to purchase. If the user is not at the point where they are ready to lay down their money, experience tells them that there is no reason to prove their identity. Ambiguous errors within the ordering flow are perhaps even more annoying. Online transactions are entirely mediated by technology, so a failure of technology at the critical point of purchase leaves users in a feedback vacuum.