Good question which had me slightly concerned for a moment; there's official documentation of the upcoming change here[0]. It does seem that this is only applied to "fulfilled by Amazon" products (and will not apply to third party sales shipped by the third party), and is an additional 5% of the existing fulfillment fee tacked on.

An example they give is of a small, standard 6-oz package where the fee will increase from $2.92 to $3.07.

[0]https://sellercentral.amazon.com/gp/help/external/GABBX6GZPA...