If you have joined an affiliate program through a network that also processes payments of products on behalf of merchants, it is not uncommon for merchants to offer customers multiple payment methods. It's great for customers, but not for affiliates. Here's an example:

Merchant X uses software distribution company Y as one of their distributors. You join company Y as an affiliate as they have a wide range of products/services from various merchants to choose from. You aren't aware that Merchant X also uses software distribution companies A,B,C for the same product - each processing payments for the merchant. The person you refer via the company Y affiliate network purchase a product from Merchant X but uses the company B payment method. The end result is no commission for you.

Before you begin advertising any products or services as an affiliate of a network that also processes orders, check the merchant site carefully. If in doubt, contact the merchant before expending any further energy, no matter how good the commission rate is. 50% of $0.00 is still zero.