Very annoying pop up in mail

Can anyone inform me how to get rid of this very annoying pop up which has started to appear in my mail?

Due to building works in my house I had to completely disconnect and store my iMac for a few days and now I have an annoying box appearing in my mail.

The box appears not just for that particular product you see, I order from them for my dog a few times each year but it also appears at other times for different emails.

Any suggestions would be welcome, thanks.

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