Hi,
I understand your requirement. I'm actually just finishing a similar project for a Prestashop module here on freelancer and I can provide you the scope of that project and a dev server where you can test that module and inspect the quality of my work. That module creates custom vouchers based on the order value, customer's currency and payment status.
What I can offer you is my full experience as a full-time senior php programmer along with my experience from freelancing where Prestashop has been my main area of interest. I can provide a fully integrated and clean prestashop module that takes advantage of the core prestashop features for maximum stability and security.
My services would include full support until the module is ready for productions, unlimited adjustments until you are happy with the result and I don't ask for upfront payments (the initial milestone is mandatory from freelancer.com)
If the project is still available please let me know as well as if you have any questions for me.
Best wishes,
Stefan