Amazon's annual Prime membership with free two-day shipping has forever changed the landscape of online shopping.

Others may occasionally offer a product at a cheaper price but Amazon knows we are impatient and want our purchases ASAP. As such, we'll pay for that $99 Prime membership in order to get our stuff in just a couple of days (or less if you're willing to pay a small fee).

Amazon's tactics have been wildly successful and continue to force the competition to either adapt or get left behind.

eBay, which has shown on multiple occasions that it's willing to modify its business model to remain relevant, is doing so once again. On Monday, the Internet pioneer announced a new program that will guarantee delivery of some 20 million eligible products in three days (with other items qualifying for one- and two-day delivery).

Hal Lawton, Senior Vice President of North America at eBay, said that while the majority of items on eBay already ship within three days or less (as well as for free), Guaranteed Delivery will give shoppers even faster delivery options and the confidence that their items will arrive on time.

The service will launch this summer at no additional cost to sellers. The perks will no doubt be enticing to sellers as it'll likely foster more sales although sellers must meet a required set of shipping standards to participate.

eBay says that if guaranteed items arrive late, the buyer can request to have the cost of shipping refunded (if shipping was free, they'll instead get a coupon towards their next eBay purchase). Optionally, they can elect to return the item at no cost.