Some packaging materials, such as those plastic sacks/bags dont provide a good surface for stamps to adhere to. This can mean you, or the post office can apply a stamp, and it drops on on-route. At the destination, the staff see it has no stamp, and the customer has to pay for the postage, and a handling fee.
Make sure the stamp will stick, if in doubt, securely tape some paper to the package and apply the stamps to that.
Forgetting to put a return/senders address on the package.
Not all parcels arrive at their destination, some get lost, or misdelivered. Without a return address, the parcel will not make its way back to you if something goes wrong.
If you bear in mind that the parcel will probably get thrown around, tossed into sacks, crushed under hundreds of other parcels, dropped from a height, and possibly get used as a football, then you will realise that proper padding and protection is a must.
Not only is it needed to protect the item, but also to protect the staff delivering it, especially if its a item that could cause injury if it cuts through the outer layers of packing.
Items should be packed so they dont knock into each other, dont rattle around in the box, and have sufficient padding around to absorb external knocks/bumps and scrapes.
If its electrical equipment, photographic equipment, or anything else that does not like moisture then consider including one of those moisture absorbing pads that you get when you buy new electrical items.
Couriers often leave large items in cold vans, or in storage depots, so the items get exposed to very cold temperatures. Bringing a cold item like this into your home will lead to condensation forming, ie moisture. Electrical equipment doesnt like this, but thankfully most people are used to knowing they should let equipment settle before switching on, so it gets to room temperature first.
If the item is fragile, it needs to be labelled FRAGILE! If you have a printer, just print of a sheet in a large font, in bold and tape it to the parcel.
Many items should only be transported one way up (copiers, printers, televisions, etc) as turning them upside down can lead to damage, so label the package clearly this way up!
Obviously many things can be posted, so I cant cover everything, but for example, lets say you wanted to post some very strong magnets....
First you would have to check with the post office to see if its allowed, if it is then you could just post them in a jiffy bag, but that could cause problems.
Why? Well the magnets themselves would probably be fine in a jiffy bag, but they are magnetic, and post goes through many machines in sorting offices... You dont want your parcel sticking to the inside of a machine and damaging it. Nor do you want your parcel sticking to a metal box in someone elses parcel. You would need to make sure the magnets are sheilded so they dont interfere or stick to anything.
Dont send things that people want in pristine condition in a normal envelope. Envelopes get crushed, bent, folded in half. If you want a peice of art to arrive in pristine condition then it needs to be packed so it cant be bent, folded or crushed.
Expensive items / Rare items / Irreplaceable items
If an item is expensive, rare, or irreplaceable then you must pay for insurance.
There are many levels of insurance, you can pay for as much coverage as you need, and you can even get consequential insurance, so if the item going missing would cause other problems you are insured against that, Always get insurance to be on the safe side, the buyer can pay for it, if they dont want it, then they cant complain if it goes missing really can they?
Dont assume that sending something by first class post, it will arrive the next day.
THIS IS NOT GUARANTEED!
If you must have next day delivery, for a urgent item, then you need to pay extra for a next day delivery service. Special delivery, or courier. Its not cheap, but its the only way to make SURE it will get there next day.
Addessing the item
Make sure the address is correct, it cant get there if its not!
Make sure you writing is ledgible, if in doubt, print the address on your printer.
Its really important to make sure that the address wont rub off, many people write on plastic packaging bags with a pen, and it rubs of really easy. Tape the address on, or tape paper on, and write it.
Not keeping records
Keep records of what you post, and when, a free proof of posting from the post office is ideal for this, and also offers limited insurance (see my other guide on free insurance for more details)
This way you can prove you sent an item if it does not arrive.
Not paying the right postage.
Dont guess, dont use cheap, innacurate scales. The buyer will end up paying for your mistakes. Dont try to save by not paying full postage, again the buyer will end up being penalised over and above the cost of the few pence you saved.
Not including some note saying who it is from
Many people buy LOTS of items on ebay, some people buy the same items from many different sellers. If a buyer buys five identical widgets from different sellers, he or she wont know who sent the widget if only a widget arrives in a blank envelope....
If your the only seller, or the items are unusual then its not as much of a problem.
Sending mail abroad has its own special considerations.
If you are sending mail outside of the european union, then you will need a customs form, not using one may result in severe delays for your customer. Customs forms are free, and easy to fill in. Be honest, as the customs may inspect the package, and its against the law to lie on a customs form.
Airmail or surface mail, whichever method you choose you should make sure the customer is aware of the length of time it may take for the item to arrive.
Just because you may offer a cheaper option, surface mail, dont assume the buyer will know it could take 7 or 8 weeks to arrive at its destination. Inform them of the timescales so they can decide which method is most suitable.
Make sure you follow the guidelines for the country you are sending to, if in doubt ask, as some items are prohibited, some countries require the return address to be written on the front, like canada. The post office is there to help in this respect.
Remember, that the list of prohibited items is there for a reason, some items cant be sent as they may actually explode on a airplane - so always make sure you declare contents properly, and if in doubt - ASK.
Dont forget additional packaging if sending mail abroad, it has a lot further to travel, and therefore more risk of damage.