Shopping days are non-negotiable but shipping days are all about how much we’re willing to pay. As for what’s more important, it really comes down to buying behaviors.

If you’re an online shopper, it’s all about the number of shipping days right now. Sure – choice and price might matter, but if you’ve got to get your gift to your sister in Cary, NC, before Christmas, a huge part of where you shop will depend on shipping guarantees.

So how are e-commerce retailers responding? And just how far will they go to maximize holiday purchases? The answers vary but it often comes down to how much consumers like you and I are willing to pay.

With mixed feelings, I’m happy to say there a ton of last minute shoppers like me. The bad news is our group is growing every year. The good news is retailers keep extending their shipping deadlines.

By continually extending shipping deadlines, I daresay retailers are enabling our delinquent shopping behavior. And good on them, they’re making lots of extra money from it!

If you’re like me and wondering who can still guarantee they will ship your gifts in time for Christmas, USA Today has a handy reference list:

•UPS. The company’s busiest day will be Wednesday, when it expects to ship 22 million packages, says Rosenberg. Its 2007 ground fuel surcharge is 3.5% and air fuel surcharge is 11.5%.

•USPS. Today is the last day to send domestic mail for arrival before Christmas, says Veto. Express Mail sent by Saturday comes with a guarantee for arrival by Christmas. Express Mail is delivered on Christmas Day, she notes.

•FedEx. Today will be FedEx’s busiest day of the year, and it expects to ship 11.3 million packages, says spokesman Howard Clabo. Saturday is the last day to ship in time for Christmas arrival.

•Wal-Mart.com. Wal-Mart’s standard shipping window expires tomorrow, Tuesday at 11:30 p.m. PT, says spokesman Ravi Jariwala.

•Amazon.com. Today is its deadline for free shipping for most orders of $25 or more, says Amazon spokesman Craig Berman. Tuesday is the deadline for standard shipping orders to arrive by Christmas Day.

•ToysRUs.com. The site extends free shipping through to tomorrow for purchases over $49, says spokeswoman Kathleen Waugh.

•Bestbuy.com. The electronics retailer’s standard shipping deadline for Christmas is Thursday at 11 a.m. ET, says Chap Achen, director of Best Buy’s order management. “The longer we can extend the cut-off date, the more likely consumers are to shop with us.”

•Circuitcity.com. The company is offering free shipping for any order over $24. The last day for standard shipping is by 1 p.m. ET Wednesday, says Mark Oldani, vice president of Circuit City Direct. The company is offering 10% off any order purchased online but picked up at the store by Saturday.

•Landsend.com. Standard shipping for Christmas ends at midnight ET Wednesday, says spokeswoman Land’s End Amanda Broderick. Monogrammed items for Christmas can be ordered through Tuesday.

DID YOU KNOW? This week, more packages will be shipped than any week of the year, says Susan Rosenberg, UPS (UPS) spokeswoman. AND – More packages, cards and letters will be mailed at the post office today — roughly 1 billion — than any day of the year, says Joanne Veto, spokeswoman for the United States Postal Service.