Braintree Payment Gateway
1) Install PaidMemberShips Pro Plugin and Set-up all Pages (That also Include all Payment Pages , billing and Invoice Pages. )
2) Go to Admin Menu Panel -> Memberships – -> Payment Galway & SSL.
In some wordpress theme in product page add to cart option is given. In this post it is explained how to change this add to cart button to Free download button to avoid trouble to user. Here it is done simply without any plugin