per amazon.com:
When you choose the option to "group my items into as few shipments as possible," we make every effort to comply with your request. This option must also be selected in order to qualify for our Free Super Saver Shipping method.
Our shipment and delivery estimates are based on a combination of factors: where we have inventory, the delivery address you provided, and how quickly we can obtain and assemble your order.
Our website clearly states that if you choose Super Saver Shipping, you can expect your order to take 3 to 5 business days longer to ship than when you choose standard shipping; this time is calculated from the time you place your order until the time it arrives, and can occur at various steps of the fulfillment process.
In order to fill your order as quickly and efficiently as possible, we may sometimes split your order into multiple shipments so we can take advantage of items in stock at different fulfillment centers.
As noted on our web site (http://www.amazon.com/supersavershipping),
to qualify for free Super Saver Shipping you must choose the option
to "Group my items into as few shipments as possible" when placing
the order.
If you change this option at any time or request a partial shipment
of the items in your order, this will invalidate the free shipping,
as your order will no longer meet all of the eligibility requirements.
For this reason, if you visit Your Account to change the shipping
option on an order, we display the following message: "If you
qualified for any of our free shipping promotions, please note that
changing your shipping option might affect your order's eligibility."