Are these sentences grammatically correct? ?

Question

The main advantage of affiliate marketing is that it is entirely performance-based. Affiliates are only paid commission once you reach their target market. Their goal is to meet the requirements of the costumers, and their marketing strategy requires less effort and drives more traffic. The output of the company is unpredictable, and it ensures that its buyers receive a quality product.  

Answer 1

Yes. They seem fine, save the typo, "costumers" which should be "customers."

Answer 2

Sounds very good to me.

Answer 3

All your sentences are correct, except your second one. Your second sentence should, to be precise, be written like this:

Affiliates are paid commission only when you reach their target market.

[Please note: the word 'only' modifies the word that immediately follows it.]