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9:26 AM PT -- A rep for Costume Agent, which originally created and distributed the controversial Caitlyn costume, tells TMZ they're not yanking it from their own site, and certainly not discontinuing it ... backlash be damned.

We're told the 'stume is one of their best sellers -- it's the #6 overall bestseller. Although they have received multiple threats over it this past week, they're standing by their product. The rep says, "It's a costume. It's funny."

A Caitlyn Jenner costume -- which harkens back to her Olympic glory days as Bruce Jenner -- is getting destroyed by critics who say it's insensitive to transgender people ... and it's already being yanked from at least one outlet.

The getup is called "I Am Cait" ... and features a decathlete's jersey with Bruce's number 935, a gold medal and those teeny tiny shorts -- all of that vintage Bruce from his 1976 decathlon victory. However, it also comes with a long-haired wig that's all Caitlyn.

eBay was flooded with criticism for selling the $44.95 costume -- several transgender charities are calling it out as mocking, instead of being an homage.

The caption for one online promo says, "Show your love for Caitlyn Jenner with this awesome adult costume."

The backlash has been enough for eBay to remove the costume, but as of now ... it's still available on Amazon.