I am in the basic stage of developing an online service, which will be sold to users around the world.Once they register with me, they will be sent their login details and a cp where they can customize their details. copy some codes to their website and they will be connected to my online service. Each of my users will get a shopping cart, i can let them take payments through paypal. But how will i go about if 1) they have their own merchant id 2) they want to go for a service like 2 checkout or autherize.net how do i connect this specific user to their choice of merchant account. Thanks
Authorize.net and 2checkout both have PHP classes available which are easy to integrate into any website. Just integrate those classes with your site. I'd like to help more but unfortunately you are being very vague with your description of the issue.