This site addresses this directly: several bonus guides are included with 3- and 6-bottle orders, and some cover adult relationship and intimacy topics; given the subject matter, this site states it is not displaying cover art for those.
This site is direct that these are educational lifestyle guides, not medical advice, that the brand does not guarantee any specific outcome tied to them, and that it has not independently reviewed their content.
Choosing not to display cover art for adult-themed material, while still disclosing that it exists, is a more restrained approach than some competitor sites in similar bonus-content categories take.