After purchasing fonts, what further "permissions" are necessary to use them in commercial design work?

There seems to be a lot of confusion surrounding font "licensing" and "permissions". Isn't the license to design with it what we are purchasing when buying the font? For basic commercial design work, what further "permissions" are needed? Are we expected to draw up a contract, send additional fees? With whom/To whom? If so, why isn't the fee schedule for basic usage clearly set forth at the time of purchase? There seems to be an across-the-board lack of clarity about this by font providers. And there is this big looming spectre of possible future lawsuits hovering over design work, a bit unfairly, I believe. The rules and requirements need to be spelled out clearly so that designers who are trying to be honest don't have to jump through hoops to get basic information, and don't have to worry about unexpectedly getting socked with some horrible fines at a future date.