If not how to get it / upgrade my package to ULTIMATE?
You need the Ultimate edition for batch rendering. See here for the detailed comparison of editions: https://boxshot.com/boxshot/order/feature-chart/
You can upgrade by just paying the difference here: https://boxshot.com/renew/ - use your current license key there and follow the instructions.
we have already purchased the Professional version of BoxShot to create Canvas images.Is it possible to call the script from our server like API integration with purchased version credentials for bulk images process one by one?
Boxshot supports command-line rendering using jobs. See here for more details: https://boxshot.com/support/boxshot-help/automation_jobs.html - batch rendering works the same way and you also need the Ultimate edition for this.
Basically, you make a reference project with the design you want to automate. Then you make a job file that links the project and artwork and finally run Boxshot to render that job for you. As long as you can automate the artwork uploading, rendering queue and processing results - you can get exactly what you want.
Note that you will need an extra license for the server then.
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