If you are confident that you can get 20k in the next couple months on a 5k mailing investment, I would go with throwing it on a card. You're going to pay some interest, but you'll still end up with a higher net income than you would if you don't do the marketing. So long as the marketing brings in the new clients.
Why do people not respect us as they do other tradesmen? Because every Tom, Dick, and Harry doesn't think he can be a plumber or electrician!