There is a known issue with the 'Add to cart' button on the product page. It is currently non-functional. We are investigating the cause and possible remedies. In the meantime, the workaround is to add the part from the main category page (e.g., Mount Bases from the Product menu). Note that adding compatible accessories from the product page works.
FIXED
Last update: 2023-05-22 : 14:43 ET