Direct mail or sales promotion device in which numbers, names or answers are invisible, even though there is no scratch-off ink covering them, but a hidden symbol can be revealed by rubbing with a coin.
The Mathematics of Direct Sales and Performance Marketing Profitability: How to Make One-Shot and Subscription-Based Products and Services Viable
Before you even begin to think about the detailed techniques of performance marketing and selling your product or service directly – the market, the design of the material, the method of order fulfillment – you …