charges with a return " before the 100 days trial"...

@flybyme,
My thoughts are the 5% / €28 is to cover the payment of a countries tax on goods, like VAT in the U.K. and Sonos would have paid that tax as part of the overall cost, but perhaps cannot recoup it - so the customer covers that loss and must recoup it themselves. Again it’s just a guess on my part, but my thoughts are Sonos use a standard 5% charge across the board for overseas transactions. 

The best thing to do here however, is to contact Sonos Sales. rather than Customer Support, as this falls within their jurisdiction and they can discuss both the refund with you privately and the reason for any charge/surcharge. Here is the link to speak to Sonos Sales:

https://support.sonos.com/s/global-sales-help