Some product owners have both options. Many pay by PayPal. Myself, I only pay by PayPal. After all, I live in a country where we don't use cheques.

Best thing to do is to check with each product owner you want to promote. Usually, as soon as you sign up for their affiliate program, you'll see your choices in the account settings. Most have a text field for entering your PayPal e-mail address, to get paid in that fashion.