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If you want to make sure holiday cards and packages arrive on time, the U.S. Postal Service reminds you to heed mailing deadlines.

It estimates it will deliver 420 million packages this holiday season — a 12 percent increase compared with last year — so adhering to deadlines is more important than ever, according to the agency.

For international destinations, mail-by dates range from Dec. 2 to Dec. 12 depending on the destination, with Global Express Guaranteed service available through Dec. 18 for most countries (Dec. 19 if mailing to Canada), the Postal Service said in a news release.

Suggested mail-by dates for cards and packages destined for U.S. addresses are: Dec. 14 for Standard Post; Dec. 20 for First-Class Mail, Dec. 21 for Priority Mail and Dec. 23 for Priority Mail Express.

The Postal Service projects Monday, Dec. 16 to be the busiest mailing day of the year. Dec. 19 should be the busiest delivery day for packages, it said.

Mailing and shippings tips include:

Confirm addresses • Print addresses clearly and be sure to include all address elements — apartment numbers, suite numbers and directional information for streets. Never guess a ZIP code. Visit usps.com to find all ZIP Codes, buy stamps and more.

Choose the right box • Select a box that is strong enough to protect the contents. Be careful when reusing boxes — they can weaken in the shipping process. Customers can buy ReadyPost boxes. Priority Mail boxes, envelopes and tubes are free.

Cushion contents with shredded or rolled newspaper, bubble wrap or foam peanuts • Popped popcorn in a bag is an eco-friendly cushion for fragile gifts. Pack contents tightly to avoid shifting during transit. Always use tape that is designed for shipping. Do not use wrapping paper or string, as they can become entangled in mail processing equipment.

Use envelopes • Lightweight paper folded into a self-mailer can cause problems for automated mail processing machines. Customers are encouraged to place such letters into an envelope before mailing. Those wanting to fold their greetings into a self-mailer should bring a sample to the Post Office to ensure it meets mailing standards.

Send gift cards in a padded envelope • Avoid putting several photographs or gift cards in an envelope with your holiday card. It makes the envelope less flexible to go through the postal system. It's best to send additional items in a separate, padded envelope.