custom coding on PHP for an exisiting open source script

This project requires to value add on an existing open source script in PHP. The exisiting off market open source script is in PHP and is for bulk email marketing. The custom requirement is to add the following: (1) Integrate bulk SMS marketing feature in the exisiting bulk email marketing script. (2) Exisitng script has the plan feature to work as Trial plan, Free plan and monthly Subscription plan. The custom requirement is to add one more plan ... Pay-as-you-Go plan so there is no need for a month on month subscription needed. We can further discuss on the existing script features and provide more access for research once we shortlist a few freelance developers.