If the possibility of mixed stock is activated within the scope of the Fulfillment by Amazon (FBA), so-called commingling transactions can appear in the monthly Amazon invoice.
What is behind these transactions is explained in this blog post.
commingling_sells: sales to Amazon, and
commingling_purchases: purchases at Amazon.
In some EU countries commingling_sells are subject to the local reverse-charge procedure (RC procedure). This means that the supplier (you) does not charge VAT because the buyer (Amazon) has to pay the VAT.
Due to different logistics structures, the local RC procedure is used in some countries (e.g. France) and not in others (e.g. Italy).