There are a number of reasons Google may require an email vs postcard verification. My experience is that the less trust Google has, the higher the chance you get the postcard option. this can include new businesses or businesses in duress/spammy categories. Trusted businesses have more verification options for the most part
New advertising option:
A review of your product or service posted by a Sterling Sky employee. This will also be shared on the Sterling Sky & LSF Twitter accounts, our Facebook group, LinkedIn, and both newsletters.