The postage is 10 [[Bank of Wurm|copper]] per item if the receiver is on the same server. You will be charged 20 copper per item if the receiver is on another server.
Items sent in containers still count, so for example a [[backpack]] with 3 [[wood scrap]]s in it would cost you 4 x 40 10 copper (1x backpack, 3x wood scrap) to send due to being counted as 4 separate items.
Mailing between servers is possible, however the Freedom Isles server is the exception. No items can be mailed to or from Freedom.