That compares to 38% of online retail sales last year with more shoppers using Amazon's one-day delivery service.
E-commerce sales rose about 18% on the previous year, GBH estimates.
Ives also estimates that Prime membership has about 88mln accounts with subscribers spending 22% more this year.
“The ring-fence that Amazon has built around its Prime customer base has significantly benefited the company in a fiercely competitive pricing environment, with much more competitive prices and inventory from Wal-Mart in particular seen over the last month, [and] with Target and Best Buy also holding their own based on our analysis and retail tracking,” the GBH note said.
More than four million people started Prime trials or started paid memberships in one week and the company’s own Echo Dot and Fire TV Stick were the top-selling products across all categories, according to Amazon.
GBH expects the near-term focus for Amazon will be on Echo and Alexa products, the Prime service, the Whole Foods integration, fulfilment investments and the Amazon Web Services (AWS) cloud subsidiary.
Ives expects AWS will wield long-term benefits by driving growth and cash flow, particularly as the company expands globally. He expects significant growth in 2018.