Should you donate the excess amount mistakenly returned to you from a foreign website, given that you cannot return it to the website and do not know if they are Muslims?

If the website returns the price of clothes you kept, then you must contact them and try to return the money. If they donate it, then it is permissible for you to take it.

However, if they do not donate it and you despair of returning it, then give it in charity on their behalf, provided that you guarantee it to them if they request it later; in which case, the charity will be counted for you. This is supported by the hadith of Ibn Mas'ud, who gave the price in charity saying, "O Allah, on behalf of the girl's master."

It does not matter that the website is owned by disbelievers, for charity benefits the disbeliever in this world, and if he embraces Islam, he will be rewarded for it in the Hereafter, as stated in the hadith of Hakim ibn Hizam: "You embraced Islam upon what good deeds you had done previously."