The Simple Membership Plugin can be easily integrated with the free WP Express Checkout plugin to accept PayPal payments for membership via a nice and secure payment popup window (the customers never leave your site during the transaction process).
The Express Checkout plugin uses the PayPal’s Express Checkout API to offer a quick checkout process using a nice popup window. It is also very effective for users making payments via their mobile devices.
This can be another membership payment option that you can add to your membership site’s Join Us page.
You create a product in the Express Checkout plugin then assign a membership level to the product in question. When customers buy this product, they will get access to that membership level.
Read this detailed step by step tutorial on their site to learn how to use the WP Express Checkout plugin to accept membership payments.