Hello
Thanks for link to existing website.
Here is short description how we will implement to get done your requirement: Export products, categories, orders etc from existing zencart store. Create template for big commerce website. Import data to bigcommere and then setup complete store.
We would require admin access to link in attach of requirement and big commerce store access details to finish with this project.
Please let us know if any questions on above.
*Please consider quote days as our working days, if you are planning for something.
Note: Milestone percentage calculation will be defined at starting of project.
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Thanks,
Dhaval