How to complain to Booking.com outside your country ( is there a centralized service team?)

 Back in May, I had an overbooking. I contacted Booking.com immediately and according to their request, I send them an accomodation alternative the same day.

Some time later, I received an invoice of €255 for the difference of price (for 4 nights!). This means that Booking.com could only arrange for an accomodation with a value of € 63.75 / night higher than mine!

The funny thing is... they are telling me that they did this because I never sent them an alternative!

Which I did. I have the proof, and I sent it to them!

What is then going on here?

Thank you for your comments and if someone knows where to send a complaint, I would be very grateful.

I tried of course to do that thhrough my extranet, but they keep telling me the same, they don't call me and I have no phone number either.