I'm selling a service online, and I'd like to advertise my website with the Pay-Per-Sale method. However, I only have a contact form (I need some information from the client and discuss what's the best solution for them) so I can't offer any tracking for the sales. Does that mean Pay-Per-Sale is not an option? Do I have to switch to Pay-Per-Lead? I can offer a pretty high payout for referrals, but the problem is: how can they trust me to pay them after they referred someone? I hope someone can help me with this problem.