Hello:
Any commercial content needs to have a customs declaration. In order to fit a typical customs declaration/ shipping label your envelope should be at least 7 x 9 inches. Put your stamps between cardboards so the envelope is stiff and they do not get damaged. The customs forms contain certain fields you need to fill out, It is that IOSS number, the number that identifies the seller/exporter (in this case ebay) for VAT purposes, for EU this number is IM2760000742. The second number is the HS code (or EU TARIC code) a 10 digit number (most EU countries use 10 digits, same may use an 8 digit number, but if you use 10 digits, it wont be a problem). That TARIC or HS number for postage stamps is: 9704000000. You enter that number like you see it. You did not say which EU country, but you may look up a particular a country and if they use an 8 digit code, you can drop the last two digits.
One can buy and print a label/customs form thru ebay. I don't know how this would work sending purchases from Sri Lanka. If not, you just need to use your post office. Hopefully, you charge the buyer enough for the postage.
