I know it's been done before.  Integrating with a payment processor is not too difficult.  The problem that you have to solve is that WebLink (with the help of the Laserfiche server) will enforce Laserfiche security but nothing else.  That is, it wasn't designed with any hooks to plug in your own access checking, which is the sort of thing you would want to keep out people who haven't paid yet.  I don't know how other integrators have addressed this.  See this post for some more discussion on the matter.