Payment gateway script development

I am the CIO of Sinteksas UAB, a 3 days old company. We are going to host multiple sites, web shop with shopping cart, from our own server. I now need to write the payment gateway script, connecting the shopping cart (Open cart at the moment, but we are open for ideas to change to a better shopping cart solution) to the bank (Swedbank Lithuania). I am personally a programmer, so i will participate directly in the project, but my skills for this job is simply not sufficient. The bank has issued a 10 pages script in English with all the requirements needed to be approved. - So we are looking for someone with experience in setting up this kind of solutions, and someone that want to work with us long term. The main goal is to have one web shop with a working shopping cart solution up and running in as little time as possible. That`s what this project is all about. Furthermore, Sinteksas going to host many sites, and have currently 1 more customer waiting. The one that we award this project to will be offered to participate in all the coming projects Sinteksas hosts for the future (and its alot :). Payment for this projected goes thru freelancer or paypal. All future projects are being paid on paypal or direct wire transfer. About the price, is it not really easy to set, as i don't really know how much time it will take. We are open for all offers, but please contact me to discuss it in details. Skype is sinteksas