More than half of U.S. taxpayers electronically file their returns. But that still leaves more than 60 million individuals who fill out their tax forms by hand and rely upon the U.S. Postal Service to get the documents to the IRS.

A large portion of those old-fashioned filers also are card-carrying members of the last-minute tax return club. That's not necessarily bad, especially if you owe the IRS. There's no need to send in your money until you absolutely have to. But you don't want to push the limit so far that you end up in a hurry and make an even more costly delivery mistake.

These tips will help ensure that you get your return into Uncle Sam's hands on time while mailing at the last possible moment:

• Call the post offices in your area to find out which will be open the latest. Use the U.S. Postal Service's office locator, which will give you post office addresses and phone numbers. The decision of how late offices stay open is made on a local level, so you need to call your local post office, says a United States Postal Service representative. "If you live in a major metropolitan area, ask for the customer relations coordinator, who can give you information on which stations will be staying open for late filers."
  
  There should be at least one post office open until midnight, if not several. Ascertain hours in advance so you won't have to guess what's open and what's not at 11:30 p.m. on the last night. Also, check to see whether any of the post office's service windows will be open through midnight in case you need to buy additional stamps or want to send your return by registered mail.
• Let your return go first class. The Internal Revenue Service recognizes the first-class postmark as proof that a return has been mailed on time. Provided your return is mailed first class and it's postmarked by the filing deadline day (April 15, unless that day falls on a weekend or federal holiday), it will be deemed "timely filed" and you won't have to pay any late charges.
• Find out where you need to mail your return. Some IRS service centers now handle returns from different parts of the country than they did previously as the agency consolidates to improve efficiency, so make sure you send your return to the proper place. The easiest solution is to use the envelope that came in your tax package. Can't find it? The mailing addresses are listed in your tax form instruction booklet. Lost that, too? Then use the IRS's interactive map to locate the service center that handles tax returns from your state. Note that addresses are slightly different (mostly ZIP code changes) depending upon which form you file.
• Use labels or print in block letters. The post office's optical character recognition systems have difficulty reading script or messy handwriting. Either use labels or neatly print, so that your return will be winging its way to Uncle Sam as quickly as possible.
• Always use a return address. That's important with all correspondence, but doubly so when dealing with the tax man. You want to make sure that if your return doesn't make it to the IRS, it does make it back to you.
• Double-check the postage. This is one time when you shouldn't skimp on stamps. The IRS won't pay the postal carrier to get your return. Insufficient postage will, at a minimum, delay your return, and it could result in the return being shipped right back to you. Avoid late penalties by weighing your return at one of your post office's self-service scales and making sure you have the correct postage.
• Bring money and stamps on your midnight express trip. Be sure to bring some cash to plug into the post office's automated stamp dispensers in case you need extra postage. Also remember that these machines like crisp bills no larger than $20 and prefer $1s and $5s. There's nothing worse than having one of these machines spitting back your tired old bills as midnight draws nigh. Some extra quarters may also come in handy in case the machine won't accept any bills. Or, if you have stamps, bring them along for the ride.
• Before your trip, make copies of all receipts and other documentation. Copies are insurance in case your return gets lost. You'll still have proof of your deductions.
• Send returns with original receipts by registered mail. While this is a good idea, make sure the option is available to you. Not all post offices will have staff on hand until midnight to process registered mail.
• As you dash into the post office, be sure to read all signs for late filers. Your post office may have special instructions for late filers. For example, it may designate a special slot just for tax returns or certain collection boxes may be slated for late pickup. (Don't worry. Even if you miss the signs, where you need to go should be pretty obvious. Just follow the crowd.) But make sure you know where to deposit your return so that it doesn't end up with an April 16 postmark.
• Kiss that envelope goodbye. Make sure that your return's envelope is sealed tight. "Contents are at risk of falling out if the envelope isn't sealed, and you'd be surprised how many people are in such a hurry that they forget to do it," the United States Postal Service representative warns.
• Late, but not last? Consider overnight mail, either the U.S. Postal Service's Express Mail or one of its competitors, such as UPS, Federal Express or DHL. Overnight mailing of tax returns has been legal since 1997. According to the IRS, the "postmark" date of an authorized private delivery service is generally the date the company records in its database or marks on the mailing label. Check with each provider about how you can get proof of this date. The benefit of overnight mailing is that generally your return can be tracked. However, it is more expensive than sending it by first class or registered mail and its deadline is sooner than mailing first-class at a post office that's open late April 15.

Follow these tips and your return, one of the millions expected to be filed at the very last minute, should reach its final destination intact and on time.