Each realm has its own bank, located in its capital with branches in any and all cities. Their primary use is to convert your bonds into gold, or vice versa. For the sake of simplicity, taxes are handled by the banks, and all the tax proceeds are given to troop leaders as bonds instead of sending out runners to deliver the physical gold. Banks can also handle transfers of gold within the realm, though the sender must be present at the bank in order to sign the necessary forms.
Bank employees are willing to handle transfers in small amounts, but the processing fees are large as a fraction of the total transfer.
Banks are the key to a smoothly running medieval economy, but still are not as developed as one might hope: there are no agreements between banks for rate of exchange of bonds, for example. The only way to send gold to another realm is to load it into a cart and physically carry it to that realm's capital, where the main branch will happily take the gold and send bonds representing it to any troop leader of that realm.
The only exception to this is that troop leaders may send bonds to their family, even when their family is in another realm or another island altogether. In this case, the troop leader sets aside a bank transfer to be picked up by an agent of the family. This agent will take care of securing the gold from the bank, and arranging for it to be transported to the family manor.