As your website says:
“ If you are registered as an entrepreneur in another EU country , we need your EU VAT identification number. To do this, please send us the following form to confirm a non-German EU VAT identification number. Pursuant to Section 13b of the German Turnover Tax Act (UStG), taxation is based on the so-called reverse charge procedure, meaning you will receive your earnings without turnover tax.”
So we should not be charged turnover tax and should receive the margin in full.
Otherwise what is the point in providing our VAT number?
In any event what is the point of setting a margin if that is not what we receive?
If you deduct VAT then the cost to us of the garment in the partner area should also be with VAT deducted
No VAT should be charged between EU businesses, that’s EU law and we both have to report it our tax authorities.