Restrict Content Pro itself does not require an SSL certificate. You may need one for your payment gateway though and with most web traffic switching to HTTPS, you should use one anyway. Stripe, PayPal, 2Checkout and Authorize.net require an SSL certificate. PayPal Standard and Express do not. I would suggest investing in an SSL certificate as soon as you can so members have confidence in your website. As you’re mixing membership and payment details, HTTP is a necessity even if payments actually go through the gateway.