Can I also use a small cardboard envelope from Amazon to send a letter via Deutsche Post?

Hello,

As a young and environmentally conscious person, I unfortunately feel like an 80-90-year-old, forced to send a letter like it was in the 1900s. Unlike older generations, I prefer to reuse things and thus protect the environment. I would like to use a small cardboard envelope I once received from Amazon for a letter. Due to the contents, a regular envelope would not provide sufficient stability. The contents weigh 130g.

Would the German Post accept this?

So the envelope would also fit in the Deutsche Post box.