Amazon said on Thursday it will offer limited free same-day delivery under its Prime shipping service, and expanded the service to San Diego and the Tampa Bay Area. Amazon offers same-day delivery to Prime members for $5.99 per order and non-members for $8.99, plus 99 cents per unit. The online retailer will allow Prime members free same-day shipping on orders over $35.

Amazon's announcement comes within days of rival Wal-Mart Stores saying it plans to test a new unlimited online shipping service this summer for $50 per year, a move that may hurt Amazon, which has an annual $99 Prime shipping service.

Google also recently launched its Express shopping option that allows retailers that do not have same-day delivery capabilities, to use its network in certain markets. Google Express costs $10 a month, or $95 a year.