The reason for not allowing users to upload vector graphics is to simplify the entire upload and printing process. By uploading graphics in PNG or JPEG (rasterised) format, they are immediately available for your personal shop. Otherwise, they would require a review, modification, and adaptation of each and every uploaded vector graphic, prolonging the times and complicating the processes.
If you need instructions on how to create an image with a transparent background, please see the instructions here.
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