Hello @Elpida Giagia 

You can either create an additionnal product, not bookable (those are the ones that you can add directly from the billing) if you absolutely want the rebooking fees to appear on the bill (don’t set it as a package) or simply modify the rate and send a new confirmation to the guest via the mailing tab of the reservation

I guess you know how to create a product right ? If not just go there: https://help.mews.com/s/article/create-or-delete-a-product?language=de

Hope this helps

Regards

S.