This depends on if you will be sending customers to your merchant pages or not. If you simply pass the customer to the merchant to enter details then you will be making use of their SSL certificate and not actually need on eof your own.
However if you are taking details on your own site then you will need an SSL certificate.
A RapidSSL certificate will be fine for such transactions. The advantage of the QuickSSL Premium is that it is also present in 99% of mobile devices therefore your customers will be able to purchase using a mobile divice such as a PDA from your site without any SSL errors.