A refund from Temu that was received by mistake

I ordered a product on the Temu website (a non-Israeli website, and apparently under non-Jewish ownership).
When the product arrived, it seemed to me that it was not good.
I reported this to the website, and they refunded me part of the money.
Afterward, I checked again and saw that I had been mistaken, and it was in fact good.
I tried to ask them to cancel the refund, but it is somewhat complicated.
Do I need to make an effort to return it to them? Or is it all right? And is this actual theft, even though they are non-Jews?
I would appreciate an urgent response.
Thank you very much!!!

Shalom u'vrachah 

You must return the money to them. 

However, you do not need to make an effort on their behalf; it is sufficient to notify them, and they will do as they wish. 

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