Let's say you're in Nek Forest and somehow you've managed to accumulate 2000gp, 2000sp, 2000cp (twenty plat? sweet!). Unfortunately you're so overburdened the seagull at the end of the dock could peck you to death and you wouldn't be able to do a thing about it. What do you do? You do your best Bay Watch slow-mo run to the mailbox. Run Hasselhof, Run!
Open the mailbox and go to the "Send" tab. Start, BUT DON'T SEND a message. Address it to yourself or an alt (just in case you do something stupid). Now, click the "Send Coin" button. Use the numbers to specify 20pp,20gp,20sp and accept the amount. DON'T SEND THE MESSAGE (hope that's clear enough). Cancel out of the message. You should hear the "ching" sound of your coin being returned to you. Now look in your inventory. You should see 20pp,20gp,20sp and you will no longer be overburdened.
This works in reverse as well. Say you have 40gp and someone offers you the Axe of Uberness for 20cp. Simply go to the mailbox, create/cancel a message to yourself containing 1cp and you'll have 30gp, 99sp, 100cp left when you look into your inventory.
hope this helps!
