Instal new payment gateway on my website - repost

My website currently uses PayPal direct as my payment gateway. I opened a account with [login to view URL] and am switching to them as my credit card processor. I need someone to instal the payment gateway API for authorize.net. I want to keep the option of paypal direct in case I decide to switch back to them. This must be completed within 2 weeks. My shopping cart is Open Cart. The person originally hired to do this failed and was unable to complete the project, i am thereby reposting it.