ok this is a very useful question and as a collector who buys/sells a lot of stuff here is what ive found;
1. Do NOT use local post office unless you absolutely have to. Their prices have gone nothing but go up and up over the years. a complete ripoff.
2. Use couriers if at all possible. The competition between the many international courier companies is fierce and they will offer discounts. dont take a price from one courier - call many !!! this is crucial. Ive seen so many people ripped off by the first courier they contact. Never use a courier from a post office - they are extremely expensive. Phone couriers or try their websites.
3. Use a courier comparison website for shipping as they get quotes from many couriers and you get the cheapest price. for example I have found nobody cheaper than;
https://www.transglobalexpress.co.uk/ Their prices are the best ! and they have a no quibble £5 insurance you can add. ANYTHING goes wrong and you get a complete refund of the postage AND the value of the item shipped. Fantastic for shipping large planes/ships which can easily get damaged during shipping.
4. Transglobal also pickup from anywhere in the world (almost) and deliver to anywhere. Very useful. DISCLAIMER: I am not employed by transglobal and have no connection to them!! I just use them a lot.
5. For mail inside your own country there is plenty of competition nowadays too. You can always find someone cheaper. In UK we have Hermes (cheaper then PO - you just need to find a nearby shop that will accept once you self print the postage label and pay online. Walk/drive down and drop it off yourself). Ebay use them as cheaper than PO. We also have Collectplus who work the same but specialise in large packages - bigger than PO will take and massively cheaper. Also very useful.
6. Ebay have a GPS - Global shipping service. If you sell something online select this option. You only pay to ship to ebay in your country - not international ! ebay pay for that and as ebay are massive they get a massive discount on shipping making the total postage cost very cheap for international. If you have something to sell to someone individually put it on ebay with GPS and a private auction and email the buyer the item number so only he can buy. Now its cheap - but you pay ebay fees but overall it still works out cheaper than you could ever post yourself.
7. Remember volume=cheap postage. The couriers only offer big discounts to those companies which use them a lot. If you know someone who works at a large company who has a big discount ask them if they can post for you. If you work for a big company use your post room and ask your boss nicely if you can pay.
8. I know business who survive on postage. They sell stuff at cost or nearly at cost but due to volume (one sells 10,000 items a week) they negotiated a big discount from couriers by playing them off against each other and they make 10k a week on the postage only - thats their profit !
Good luck,
Cheers,
Stefan
Palmerston North, NZ (but stuck in London