Best way to offer domain mapping as a single service?

I need to offer my users the chance to purchase domain mapping as a one-off service, per site. I’m thinking Pro Sites is the obvious way to do this, but I don’t want to offer it at signup, I want to offer it after they’ve created a free site. And, as I mentioned, I want to offer it as a one-off, and not necessarily part of a package (although I would like to also make it part of a Pro package eventually). Anyone have input on the best way to accomplish this? I’m new to Pro Sites so I may be missing something obvious here.