Sellers who are new to international shipping often come to the forums for advice, but I prefer asking Royal Mail.
This is what Royal Mail say about addressing international mail. At first they say put sender's address on the back ... but read on. They state further down that if you put a customs declaration sticker on your parcel, you MUST include seller's full name and address on the FRONT. Click on the picture below to enlarge it to readable proportions. The right hand image shows the instructions on the reverse of the form.
I did once have a parcel returned to me because my sender's address was confused by the post office (thanks guys) and this is why people often decide to put their return address on the back. The instructions here though are definitely for the front. I got an address stamper (free from vistaprint) so I stamp my address at the top left hand corner under the airmail sticker, then write the buyer's address good and big!
A frequently-asked question is whether it's ok to mark the parcel as "gift". The answer is that it is only ok if the contents are indeed a gift, from you to the recipient. If you have sold the contents of the parcel, then it is NOT a gift. Marking a package as "gift" when it is not is fraud and carries a big penalty which I cannot find - help anyone?
Worryingly it seems the buyer can also be held responsible if the seller inaccurately represents the contents - see below or here
"2.1 Does the sender have to declare the goods?
Yes. Under international postal agreements the sender must complete a customs declaration (form CN22 or CN23) which in most cases should be fixed to the package. The declaration includes a description of the goods, the value and whether they are gifts or commercial items. Any Post Office abroad should be able to give advice to the sender.
Under customs law, you as the importer are legally responsible for the information on the declaration; therefore it is in your own interest to ensure, wherever possible, that the sender abroad completes the declaration accurately and in full.
If no declaration is made, or the information is inaccurate, the package may be delayed while the Border Force make further enquiries, or in some cases the package and its contents may be returned to the sender or seized by the Border Force."
So - what to write on the form? It asks for "detailed description", but what does that really mean? I assume it's so that if the parcel is opened at customs the contents will match the description. I write "knitted and wired soft sculptures" on mine. I started to add "wired" because I thought maybe parcels might go through some sort of security scan at airports (no idea if they do!) and if I'd only written "knitted" maybe alarms would go off. Anyone know?
Value is another interesting question. The lower limit for parcels coming into the UK is a daft £15, at which point you might well get charged VAT and an £8 fee from the post office for their hard work collecting that tax.
Buyers sometimes ask for the price to be fraudulently stated as below the threshold. What happens if that parcel then gets lost in the system? No chance of claiming back the true price!
Lastly - if you ask at your post office they will give you a roll of customs and airmail stickers so you can fill them in at home and save time in that queue!