Did you know you can mail anything that will take a stamp and weighs less than 13ounces without a box? Why would you want to mail something awkward? An invitation to Christmas celebrated at the beach is far more exciting when you're mailed a pail and shovel!
Although it hasn't been a secret that's been kept by the postal system, mailing any item that's less than 108" in combined length and girth doesn't require a box or envelope. It's a small fee (depending on the size) but the possibilities are endless.
Over at The Giver's Log, they've been busy cataloging the items they've mailed in the past, in fact they've even dedicated a Flickr pool to such things and although most items are just for fun, they could easily be transformed into an invitation of symbol of holiday well wishes. Besides, who wants a card hanging around when you could have any number of things that might have actual purpose instead of sitting there and looking pretty.
Try printing your invitation or greetings on sticker paper and attach them to something simple like a box of candy. If you're feeling daring, use an old pop bottle and fill it with assorted treasures, an actual invitation, glitter, snacks or photos. Think outside the box and check out a few ideas over at The Giver's Log.
Image: The Giver's Log

Sprout Side Table
In the carribean, they have stands setup with tropically painted coconuts, for tourists. They paint the mailing address right onto the coconut and drop it in the mail. Our local mailman said that they had so much fun receiving the coconut. Mailed it to my mom and she loved it. Great conversation piece.
I used to mail letters in walnut shells when I was a teenager. I would crack the nut carefully, remove the meat, write my letter in tiny writing on a strip of paper, roll it up, tuck it inside, glue the walnut back together, and stick on an address and stamp. But that was decades ago. I think there are now lower limits to the size of packages and walnuts no longer work.
yeah nooo... I don't think I'll be sending 50 pieces of disposable plastic for my holiday party...
I like the more eco-friendly ideas though
Wired magazine used to ask readers to mail them crazy items. Each month they would feature one item. It was a pretty cool feature, and I'm sorry they stopped running it.
There is tail (told to me first had by the Portland OR, postal director) that someone once mailed an egg. It arrive un-cracked.
tail = tale.
When I first moved away from home, I used to send my little sister rubber balls and frisbees with messages written all over them. She loved them!
Has anyone received or sent anything like this lately?
I sent about 50 chocolate chip cookies, each in a plastic bag, a couple of years ago. I wrote the name and address on the bag. They all arrived--some in smithereens and some whole.
The original idea presented in this post is incredibly wasteful. You're essentially sending trash through the mail. Cute idea - needs to be used more thoughtfully.
I'm a former postal employee. My job (in SLC) was to read the most exasperating and unreadable mail in the country. The post office will deliver a lot of things because they are required by federal law to make their best effort. That they are required to deliver crap like this doesn't make it preferable or cost efficient. Mailing a pail would be a particularly bad idea because other pieces of mail could get trapped inside it. Those mail pieces could then be sent to the wrong city, state, or even country. Lots of people like to get creative with their mail. Doodles. Stickers. Crazy fonts. Those "cute" little return stamps that go in a circle. (How can a machine read something that goes around in a circle?) If you want your mail to be cute, make it cute on the inside of the package. The outside should be plain and boring as can be. (No stickers or doodles or additional messages please.) The destination address should be neatly printed or printed using a good sized font (Arial or Times New Roman are good) that isn't smudged. Not too small or too large. The return address goes on the top left hand corner. Not the bottom. Not the back. Not anywhere else. The envelope or box shouldn't contain unsealed crevices where other mail can get trapped and rerouted to the wrong location. People learn how to address mail in the 3rd grade (or at least I did) and then they seem to forget. Or they feel that they have to let their creative juices flow. The post office is struggling to stay solvent and ideas like this don't help. Not to be a party pooper, but if you do stuff like this, please don't complain the next time the price of postage goes up.
I started delivering mail nearly 10 years ago. We mostly see letters and parcels but we see some strange things. For example, on occasion we have live baby chicks in the post office with plenty of chirping noises, letter sized envelopes with artistic drawings on them in pencil possibly from incarcerated people, (one time there was a map showing where to deliver the letter) advertising on a paper bag and a key placed in a small tube (hundreds of tubes in one day) promoting a auto dealership and others. Sometimes we have something that plays music in a parcel that will play off and on.
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My sister just retired for the PO and I went with her to mail a photograph to her son. She rolled it up and stuck it inside a plastic soda pop bottle! I since have mailed items/letters/invitations the same way.