We don't yet have upload categories for the different stamp companies. However, you can include the other companies' names in the Keyword section when you upload. That will allow your card to show up in the other companies' galleries.
Keep in mind that if the company has two or more words in its name, you should either run the words together, or join them with an underscore, like these examples:
PurpleOnionDesigns
Lizzie_Anne_Designs
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