This is an elliptical construction with to missing. It is widely understood that
Why don't you go pay for them?
means the following:
Why don't you go to pay for them?
Too often for my taste, people say
Why don't you go and pay for them?
Strictly speaking, this isn't what is literally meant, but people say it often enough that everyone knows that it means the same thing. Don't use it in formal English.
In summary, all three sentences mean the same thing to an English listener. The first sentence is just a shorter way to say the same thing as the second, and the last sentence is less formal but means the same thing.