Assuming there is a receipt available, which there should be for an online purchase, this is no different from any other supplier transaction. When you say that your finance regs do not allow this, do you mean they don't include provision for it, or do they expressly forbid it? If the former, I'd just pay it on the basis that they don't say you can't do it. I allow members of my councils to make purchases and reclaim the cost where there is a valid reason to do so.