Bag of Sending
The Bag of Sending is an item obtained from the Ambitious Solen Queen quest. It allows you to pass items to your bank box, and is especially useful for sending gold collected in Feluccan Dungeons due to it's weight. The bags are charged with Translocation Powder obtained both through the same quest and from the Solen Matriarch, though they eventually become "saturated" and no longer accept recharging. At their maximum they may hold 30 charges.
Revamp
Originally bags used one charge per item sent, but during Publish 48 changes were made to the cost of transporting items. The formula is now 1 charge for every 10 stones of Weight rounded up. For example, if you tried to send a 35 stone item, it will cost 4 charges. This made it impractical to send Gold (as every 50 pieces weigh a Stone each, only 15000 gold can be sent with a fully charged bag!).
In a November of 2007 Five on Friday, it was stated that slowing gold collection from monsters was indeed the primary reason for the change.