How do Artfinder's 'Free Shipping Promotions' work?

Every once in a while, usually on special occasions (Christmas, Easter, International Chocolate Day, etc), Artfinder will run a 'Free Shipping Promotion'.

Shipping will be subsidised in full by Artfinder for all artworks for which the seller's shipping charge is less than 15% of the artwork retail price. For all artworks with associated shipping charges exceeding this threshold, the full shipping charge will be applied. The total displayed at checkout will reflect the full amount payable, with shipping removed where applicable.

Please note, the promotion works on a product-by-product basis meaning that it is the individual product shipping cost  vs individual product price that is used in the 15% threshold calculation, not the total basket value.