Currently there seems to be great confusion around what the different Royal Mail overseas rates mean and what you can and cannot send via them. This confusion, unfortunately extends to counter staff with differing advice/rates being quoted.
However, the rules ARE quite simple!
Think of it this way:-
There are three main overseas rates. They are : Letter, Small Packet and Printed Papers. Anything can then be sent via Air (Plane) or Surface (Boat)...
But if you think of them, as 'Standard Rate", "Concessionary Rate 1" and "Concessionary Rate 2" then you are on the right lines!
It goes like this:-
Letter Rate (Otherwise known in our new world as Standard Rate) :- ANYTHING (Unless it is on the prohibited or restricted list either generally or specifically for the country you are sending to) and within the size limits can be sent via letter rate. It does NOT have to have a letter in it! I am convinced that there is this notion in some peoples heads that there letters weighing 2kgs are whizzing around the world !
Small Packet Rate (In our new world, just for this guide, its now 'Concessionary Rate 1') -You can send anything that is gifts, goods or commercial samples. You can also include a letter relating to the contents. This then would include everything sold on Ebay..
Printed Papers (In our new world, just for this guide, its now 'Concessionary Rate 2') - You can send books, magazines, newspapers, leaflets and pamphlets abroad. You can include a letter relating to the contents.
Note:- The Size limits for all items sent by Royal mail International is the same whether the item is a Letter, Small Packet or Printed Paper. -Length, width, depth combined: 90cm Greatest single dimension: 60cm
There is also a minimum size:-One surface at least 9cm x 14cm.
For items in the form of a roll:-Length plus twice diameter: 14cm Length: 90cm (Minimum Length plus twice diameter: 17cm Greatest single dimension: 10cm)
So the question then becomes 'Why send anything 'Standard' (Letter) Rate if there are concessionary rates that seem to cover nearly everything? Well, In a few countries 'letters' are delivered before the 'small packets' say on a first delivery rather than a later one, but in general there is no difference in delivery time as the items travel in the same mail stream.
So, apart from where its cheaper to send an item letter rate (for items upto 40g to Europe and 80g Worldwide) , then make full use of the 'concessionary rates' !
If you wish to send a 'Letter' -(thats a written one , not our 'Standard Rate') then I would suggest a Airletter ( Aerogram)... seperatly.
Also there are some weights where the Small Packet rate is cheaper than the Printed Paper rate, in this case you could send the book as 'goods' and make use of that rate instead .
Markings - Items should be marked 'Small Packet' or ;'Printed Papers' as appropriate.
Customs- All items including 'Goods' send to destinations outside the EU should have the appropriate customs declaration affixed (generally a CN22). This includes items being sent letter rate!- There is again some misunderstanding regarding this, as some people belive that by sending something letter rate a customs declaration is suddenly not required!
The rule is - Goods Included & Outside EU = Customs Declaration required.
It is usually acceptable to just write 'Printed Papers' on items being sent via that rate (with no customs declaration) but there are a couple of exceptions.
In all cases check at Royalmail.com if you are unsure.
[This guide is my own personal interpretation of the current rules and should not be relied on as authoritative.]