How would you handle the transfer of large quantities of gold in a low-magic, medieval fantasy setting?
I'm am running a campaign based on a homebrew variation of Pathfinder in which magic use carries grave risks. I've been able to adapt traditional D & D tropes to work in this context rather well but I am at a loss for providing players with a simple system to access their gold while traveling. In previous games I've solved this problem through magic (i.e bills of exchange that are verified through zone of truth, a bag of holding etc.) but that won't work in this setting. This has suddenly become a pressing issue because one of the players stumbled into a decent sized fortune.
Two obvious approaches that I've rejected are simply forcing them to carry only a reasonable amount of gold and hide the rest somewhere or having them cart their gold around with them--and hire guards to protect it. Both of these ideas are reasonable, and feel realistic, but they are too burdensome for this particular campaign. Maybe I'm being too nice, but I would like the players' assets to reasonably easy for them to manage.
My current plan is to introduce bills of exchange that can be negotiated at each major city they visit. But I don't know how to handle player questions like "what prevents forgery?" or "how does the bank make money?" I am wondering if other GMs have run into these questions and problems and come up with better solutions than this.