Have a llok at this file:
http://esodata.uesp.net/current/src/...lsend.lua.html
It includes the mail send COD money box and is using some functions like
ZO_DefaultMoneyInputField_SetUsePlayerGoldAsMax
Maybe there is an edit field with the type ZO_DefaultMoneyInputField then, or you can use the same functions to provide an input field with callback functions and maximuma moutn = player gold.