I'm interested in that project. I'm experienced (15+) perl web developer and linux administrator. I've done xfs payment integration before, even on freelancer.com.
Redirecting user using post method is much more complicated that just redirecting to payment gateway with ref key attached. The gateway must allow cross-site posting, otherwise you have to fetch post data provided by user and repost to payment gateway - which means you receive card infor data given by user. But i do not know the gateway configuration.
providing it is required before i accept a project, as it's needed to accurately estimate the hours needed to to the job, which influence the project cost.
Regards.
piotr