Thanks for your interest and hard work, but none of this applies to an existing Merchant Account. In fact using an existing Merchant Account means a extra expense etc. I have come to the conclusion that the use of forms is by far the best thing to use. A system that has been around since 1996. I must admit that I am disappointed in the fact that none of these that I have investigated will allow you to email the request with credit card.
Yes most of the eCommerce carts that we offer have the ability to accept credit cards. We have a collection of articles that describe how to setup the payment module or options for OsCommerce, ZenCart, CubeCart, OpenCart, and the Premium Website Builder eShop that I would recommend checking out.
You would need to use a merchant account such as from Authorize.net or PayPal in order to accept credit cards via OpenCart. What is the existing merchant account that you currently are using that you are trying to link up to the OpenCart site? You might need to get in contact with them, and find out if there is a way to use their system with OpenCart.